text: /*
   * Copyright (c) 2003-2011 The DragonFly Project.  All rights reserved.
   *
   * This code is derived from software contributed to The DragonFly Project
   * by Matthew Dillon <dillon@backplane.com>
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   *
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in
   *    the documentation and/or other materials provided with the
   *    distribution.
   * 3. Neither the name of The DragonFly Project nor the names of its
   *    contributors may be used to endorse or promote products derived
   *    from this software without specific, prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
   * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
   * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
   * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
   * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
   * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   * SUCH DAMAGE.
   */
  
  /*
   * Each cpu in a system has its own self-contained light weight kernel
   * thread scheduler, which means that generally speaking we only need
   * to use a critical section to avoid problems.  Foreign thread 
   * scheduling is queued via (async) IPIs.
   */
  
  #include <sys/param.h>
  #include <sys/systm.h>
  #include <sys/kernel.h>
  #include <sys/proc.h>
  #include <sys/rtprio.h>
  #include <sys/kinfo.h>
  #include <sys/queue.h>
  #include <sys/sysctl.h>
  #include <sys/kthread.h>
  #include <machine/cpu.h>
  #include <sys/lock.h>
  #include <sys/spinlock.h>
  #include <sys/ktr.h>
  
  #include <sys/thread2.h>
  #include <sys/spinlock2.h>
  #include <sys/mplock2.h>
  
  #include <sys/dsched.h>
  
  #include <vm/vm.h>
  #include <vm/vm_param.h>
  #include <vm/vm_kern.h>
  #include <vm/vm_object.h>
  #include <vm/vm_page.h>
  #include <vm/vm_map.h>
  #include <vm/vm_pager.h>
  #include <vm/vm_extern.h>
  
  #include <machine/stdarg.h>
  #include <machine/smp.h>
  
  #if !defined(KTR_CTXSW)
  #define KTR_CTXSW KTR_ALL
  #endif
  KTR_INFO_MASTER(ctxsw);
  KTR_INFO(KTR_CTXSW, ctxsw, sw, 0, "#cpu[%d].td = %p", int cpu, struct thread *td);
  KTR_INFO(KTR_CTXSW, ctxsw, pre, 1, "#cpu[%d].td = %p", int cpu, struct thread *td);
  KTR_INFO(KTR_CTXSW, ctxsw, newtd, 2, "#threads[%p].name = %s", struct thread *td, char *comm);
  KTR_INFO(KTR_CTXSW, ctxsw, deadtd, 3, "#threads[%p].name = <dead>", struct thread *td);
  
  static MALLOC_DEFINE(M_THREAD, "thread", "lwkt threads");
  
  #ifdef	INVARIANTS
  static int panic_on_cscount = 0;
  #endif
  static __int64_t switch_count = 0;
  static __int64_t preempt_hit = 0;
  static __int64_t preempt_miss = 0;
  static __int64_t preempt_weird = 0;
  static int lwkt_use_spin_port;
  static struct objcache *thread_cache;
  
  static void lwkt_schedule_remote(void *arg, int arg2, struct intrframe *frame);
  static void lwkt_setcpu_remote(void *arg);
  
  extern void cpu_heavy_restore(void);
  extern void cpu_lwkt_restore(void);
  extern void cpu_kthread_restore(void);
  extern void cpu_idle_restore(void);
  
  /*
   * We can make all thread ports use the spin backend instead of the thread
   * backend.  This should only be set to debug the spin backend.
   */
  TUNABLE_INT("lwkt.use_spin_port", &lwkt_use_spin_port);
  
  #ifdef	INVARIANTS
  SYSCTL_INT(_lwkt, OID_AUTO, panic_on_cscount, CTLFLAG_RW, &panic_on_cscount, 0,
      "Panic if attempting to switch lwkt's while mastering cpusync");
  #endif
  SYSCTL_QUAD(_lwkt, OID_AUTO, switch_count, CTLFLAG_RW, &switch_count, 0,
      "Number of switched threads");
  SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_hit, CTLFLAG_RW, &preempt_hit, 0, 
      "Successful preemption events");
  SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_miss, CTLFLAG_RW, &preempt_miss, 0, 
      "Failed preemption events");
  SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_weird, CTLFLAG_RW, &preempt_weird, 0,
      "Number of preempted threads.");
  static int fairq_enable = 0;
  SYSCTL_INT(_lwkt, OID_AUTO, fairq_enable, CTLFLAG_RW,
  	&fairq_enable, 0, "Turn on fairq priority accumulators");
  static int fairq_bypass = -1;
  SYSCTL_INT(_lwkt, OID_AUTO, fairq_bypass, CTLFLAG_RW,
  	&fairq_bypass, 0, "Allow fairq to bypass td on token failure");
  extern int lwkt_sched_debug;
  int lwkt_sched_debug = 0;
  SYSCTL_INT(_lwkt, OID_AUTO, sched_debug, CTLFLAG_RW,
  	&lwkt_sched_debug, 0, "Scheduler debug");
  static int lwkt_spin_loops = 10;
  SYSCTL_INT(_lwkt, OID_AUTO, spin_loops, CTLFLAG_RW,
  	&lwkt_spin_loops, 0, "Scheduler spin loops until sorted decon");
  static int lwkt_spin_reseq = 0;
  SYSCTL_INT(_lwkt, OID_AUTO, spin_reseq, CTLFLAG_RW,
  	&lwkt_spin_reseq, 0, "Scheduler resequencer enable");
  static int lwkt_spin_monitor = 0;
  SYSCTL_INT(_lwkt, OID_AUTO, spin_monitor, CTLFLAG_RW,
  	&lwkt_spin_monitor, 0, "Scheduler uses monitor/mwait");
  static int lwkt_spin_fatal = 0;	/* disabled */
  SYSCTL_INT(_lwkt, OID_AUTO, spin_fatal, CTLFLAG_RW,
  	&lwkt_spin_fatal, 0, "LWKT scheduler spin loops till fatal panic");
  static int preempt_enable = 1;
  SYSCTL_INT(_lwkt, OID_AUTO, preempt_enable, CTLFLAG_RW,
  	&preempt_enable, 0, "Enable preemption");
  static int lwkt_cache_threads = 0;
  SYSCTL_INT(_lwkt, OID_AUTO, cache_threads, CTLFLAG_RD,
  	&lwkt_cache_threads, 0, "thread+kstack cache");
  
  static __cachealign int lwkt_cseq_rindex;
  static __cachealign int lwkt_cseq_windex;
  
  /*
   * These helper procedures handle the runq, they can only be called from
   * within a critical section.
   *
   * WARNING!  Prior to SMP being brought up it is possible to enqueue and
   * dequeue threads belonging to other cpus, so be sure to use td->td_gd
   * instead of 'mycpu' when referencing the globaldata structure.   Once
   * SMP live enqueuing and dequeueing only occurs on the current cpu.
   */
  static __inline
  void
  _lwkt_dequeue(thread_t td)
  {
      if (td->td_flags & TDF_RUNQ) {
  	struct globaldata *gd = td->td_gd;
  
  	td->td_flags &= ~TDF_RUNQ;
  	TAILQ_REMOVE(&gd->gd_tdrunq, td, td_threadq);
  	--gd->gd_tdrunqcount;
  	if (TAILQ_FIRST(&gd->gd_tdrunq) == NULL)
  		atomic_clear_int(&gd->gd_reqflags, RQF_RUNNING);
      }
  }
  
  /*
   * Priority enqueue.
   *
   * There are a limited number of lwkt threads runnable since user
   * processes only schedule one at a time per cpu.  However, there can
   * be many user processes in kernel mode exiting from a tsleep() which
   * become runnable.
   *
   * NOTE: lwkt_schedulerclock() will force a round-robin based on td_pri and
   *	 will ignore user priority.  This is to ensure that user threads in
   *	 kernel mode get cpu at some point regardless of what the user
   *	 scheduler thinks.
   */
  static __inline
  void
  _lwkt_enqueue(thread_t td)
  {
      thread_t xtd;
  
      if ((td->td_flags & (TDF_RUNQ|TDF_MIGRATING|TDF_BLOCKQ)) == 0) {
  	struct globaldata *gd = td->td_gd;
  
  	td->td_flags |= TDF_RUNQ;
  	xtd = TAILQ_FIRST(&gd->gd_tdrunq);
  	if (xtd == NULL) {
  	    TAILQ_INSERT_TAIL(&gd->gd_tdrunq, td, td_threadq);
  	    atomic_set_int(&gd->gd_reqflags, RQF_RUNNING);
  	} else {
  	    /*
  	     * NOTE: td_upri - higher numbers more desireable, same sense
  	     *	     as td_pri (typically reversed from lwp_upri).
  	     *
  	     *	     In the equal priority case we want the best selection
  	     *	     at the beginning so the less desireable selections know
  	     *	     that they have to setrunqueue/go-to-another-cpu, even
  	     *	     though it means switching back to the 'best' selection.
  	     *	     This also avoids degenerate situations when many threads
  	     *	     are runnable or waking up at the same time.
  	     *
  	     *	     If upri matches exactly place at end/round-robin.
  	     */
  	    while (xtd &&
  		   (xtd->td_pri >= td->td_pri ||
  		    (xtd->td_pri == td->td_pri &&
  		     xtd->td_upri >= td->td_upri))) {
  		xtd = TAILQ_NEXT(xtd, td_threadq);
  	    }
  	    if (xtd)
  		TAILQ_INSERT_BEFORE(xtd, td, td_threadq);
  	    else
  		TAILQ_INSERT_TAIL(&gd->gd_tdrunq, td, td_threadq);
  	}
  	++gd->gd_tdrunqcount;
  
  	/*
  	 * Request a LWKT reschedule if we are now at the head of the queue.
  	 */
  	if (TAILQ_FIRST(&gd->gd_tdrunq) == td)
  	    need_lwkt_resched();
      }
  }
  
  static __boolean_t
  _lwkt_thread_ctor(void *obj, void *privdata, int ocflags)
  {
  	struct thread *td = (struct thread *)obj;
  
  	td->td_kstack = NULL;
  	td->td_kstack_size = 0;
  	td->td_flags = TDF_ALLOCATED_THREAD;
  	td->td_mpflags = 0;
  	return (1);
  }
  
  static void
  _lwkt_thread_dtor(void *obj, void *privdata)
  {
  	struct thread *td = (struct thread *)obj;
  
  	KASSERT(td->td_flags & TDF_ALLOCATED_THREAD,
  	    ("_lwkt_thread_dtor: not allocated from objcache"));
  	KASSERT((td->td_flags & TDF_ALLOCATED_STACK) && td->td_kstack &&
  		td->td_kstack_size > 0,
  	    ("_lwkt_thread_dtor: corrupted stack"));
  	kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
  	td->td_kstack = NULL;
  	td->td_flags = 0;
  }
  
  /*
   * Initialize the lwkt s/system.
   *
   * Nominally cache up to 32 thread + kstack structures.  Cache more on
   * systems with a lot of cpu cores.
   */
  void
  lwkt_init(void)
  {
      TUNABLE_INT("lwkt.cache_threads", &lwkt_cache_threads);
      if (lwkt_cache_threads == 0) {
  	lwkt_cache_threads = ncpus * 4;
  	if (lwkt_cache_threads < 32)
  	    lwkt_cache_threads = 32;
      }
      thread_cache = objcache_create_mbacked(
  				M_THREAD, sizeof(struct thread),
  				0, lwkt_cache_threads,
  				_lwkt_thread_ctor, _lwkt_thread_dtor, NULL);
  }
  
  /*
   * Schedule a thread to run.  As the current thread we can always safely
   * schedule ourselves, and a shortcut procedure is provided for that
   * function.
   *
   * (non-blocking, self contained on a per cpu basis)
   */
  void
  lwkt_schedule_self(thread_t td)
  {
      KKASSERT((td->td_flags & TDF_MIGRATING) == 0);
      crit_enter_quick(td);
      KASSERT(td != &td->td_gd->gd_idlethread,
  	    ("lwkt_schedule_self(): scheduling gd_idlethread is illegal!"));
      KKASSERT(td->td_lwp == NULL ||
  	     (td->td_lwp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
      _lwkt_enqueue(td);
      crit_exit_quick(td);
  }
  
  /*
   * Deschedule a thread.
   *
   * (non-blocking, self contained on a per cpu basis)
   */
  void
  lwkt_deschedule_self(thread_t td)
  {
      crit_enter_quick(td);
      _lwkt_dequeue(td);
      crit_exit_quick(td);
  }
  
  /*
   * LWKTs operate on a per-cpu basis
   *
   * WARNING!  Called from early boot, 'mycpu' may not work yet.
   */
  void
  lwkt_gdinit(struct globaldata *gd)
  {
      TAILQ_INIT(&gd->gd_tdrunq);
      TAILQ_INIT(&gd->gd_tdallq);
  }
  
  /*
   * Create a new thread.  The thread must be associated with a process context
   * or LWKT start address before it can be scheduled.  If the target cpu is
   * -1 the thread will be created on the current cpu.
   *
   * If you intend to create a thread without a process context this function
   * does everything except load the startup and switcher function.
   */
  thread_t
  lwkt_alloc_thread(struct thread *td, int stksize, int cpu, int flags)
  {
      static int cpu_rotator;
      globaldata_t gd = mycpu;
      void *stack;
  
      /*
       * If static thread storage is not supplied allocate a thread.  Reuse
       * a cached free thread if possible.  gd_freetd is used to keep an exiting
       * thread intact through the exit.
       */
      if (td == NULL) {
  	crit_enter_gd(gd);
  	if ((td = gd->gd_freetd) != NULL) {
  	    KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK|
  				      TDF_RUNQ)) == 0);
  	    gd->gd_freetd = NULL;
  	} else {
  	    td = objcache_get(thread_cache, M_WAITOK);
  	    KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK|
  				      TDF_RUNQ)) == 0);
  	}
  	crit_exit_gd(gd);
      	KASSERT((td->td_flags &
  		 (TDF_ALLOCATED_THREAD|TDF_RUNNING|TDF_PREEMPT_LOCK)) ==
  		 TDF_ALLOCATED_THREAD,
  		("lwkt_alloc_thread: corrupted td flags 0x%X", td->td_flags));
      	flags |= td->td_flags & (TDF_ALLOCATED_THREAD|TDF_ALLOCATED_STACK);
      }
  
      /*
       * Try to reuse cached stack.
       */
      if ((stack = td->td_kstack) != NULL && td->td_kstack_size != stksize) {
  	if (flags & TDF_ALLOCATED_STACK) {
  	    kmem_free(&kernel_map, (vm_offset_t)stack, td->td_kstack_size);
  	    stack = NULL;
  	}
      }
      if (stack == NULL) {
  	stack = (void *)kmem_alloc_stack(&kernel_map, stksize);
  	flags |= TDF_ALLOCATED_STACK;
      }
      if (cpu < 0) {
  	cpu = ++cpu_rotator;
  	cpu_ccfence();
  	cpu %= ncpus;
      }
      lwkt_init_thread(td, stack, stksize, flags, globaldata_find(cpu));
      return(td);
  }
  
  /*
   * Initialize a preexisting thread structure.  This function is used by
   * lwkt_alloc_thread() and also used to initialize the per-cpu idlethread.
   *
   * All threads start out in a critical section at a priority of
   * TDPRI_KERN_DAEMON.  Higher level code will modify the priority as
   * appropriate.  This function may send an IPI message when the 
   * requested cpu is not the current cpu and consequently gd_tdallq may
   * not be initialized synchronously from the point of view of the originating
   * cpu.
   *
   * NOTE! we have to be careful in regards to creating threads for other cpus
   * if SMP has not yet been activated.
   */
  static void
  lwkt_init_thread_remote(void *arg)
  {
      thread_t td = arg;
  
      /*
       * Protected by critical section held by IPI dispatch
       */
      TAILQ_INSERT_TAIL(&td->td_gd->gd_tdallq, td, td_allq);
  }
  
  /*
   * lwkt core thread structural initialization.
   *
   * NOTE: All threads are initialized as mpsafe threads.
   */
  void
  lwkt_init_thread(thread_t td, void *stack, int stksize, int flags,
  		struct globaldata *gd)
  {
      globaldata_t mygd = mycpu;
  
      bzero(td, sizeof(struct thread));
      td->td_kstack = stack;
      td->td_kstack_size = stksize;
      td->td_flags = flags;
      td->td_mpflags = 0;
      td->td_type = TD_TYPE_GENERIC;
      td->td_gd = gd;
      td->td_pri = TDPRI_KERN_DAEMON;
      td->td_critcount = 1;
      td->td_toks_have = NULL;
      td->td_toks_stop = &td->td_toks_base;
      if (lwkt_use_spin_port || (flags & TDF_FORCE_SPINPORT))
  	lwkt_initport_spin(&td->td_msgport, td);
      else
  	lwkt_initport_thread(&td->td_msgport, td);
      pmap_init_thread(td);
      /*
       * Normally initializing a thread for a remote cpu requires sending an
       * IPI.  However, the idlethread is setup before the other cpus are
       * activated so we have to treat it as a special case.  XXX manipulation
       * of gd_tdallq requires the BGL.
       */
      if (gd == mygd || td == &gd->gd_idlethread) {
  	crit_enter_gd(mygd);
  	TAILQ_INSERT_TAIL(&gd->gd_tdallq, td, td_allq);
  	crit_exit_gd(mygd);
      } else {
  	lwkt_send_ipiq(gd, lwkt_init_thread_remote, td);
      }
      dsched_new_thread(td);
  }
  
  void
  lwkt_set_comm(thread_t td, const char *ctl, ...)
  {
      __va_list va;
  
      __va_start(va, ctl);
      kvsnprintf(td->td_comm, sizeof(td->td_comm), ctl, va);
      __va_end(va);
      KTR_LOG(ctxsw_newtd, td, td->td_comm);
  }
  
  /*
   * Prevent the thread from getting destroyed.  Note that unlike PHOLD/PRELE
   * this does not prevent the thread from migrating to another cpu so the
   * gd_tdallq state is not protected by this.
   */
  void
  lwkt_hold(thread_t td)
  {
      atomic_add_int(&td->td_refs, 1);
  }
  
  void
  lwkt_rele(thread_t td)
  {
      KKASSERT(td->td_refs > 0);
      atomic_add_int(&td->td_refs, -1);
  }
  
  void
  lwkt_free_thread(thread_t td)
  {
      KKASSERT(td->td_refs == 0);
      KKASSERT((td->td_flags & (TDF_RUNNING | TDF_PREEMPT_LOCK |
  			      TDF_RUNQ | TDF_TSLEEPQ)) == 0);
      if (td->td_flags & TDF_ALLOCATED_THREAD) {
      	objcache_put(thread_cache, td);
      } else if (td->td_flags & TDF_ALLOCATED_STACK) {
  	/* client-allocated struct with internally allocated stack */
  	KASSERT(td->td_kstack && td->td_kstack_size > 0,
  	    ("lwkt_free_thread: corrupted stack"));
  	kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
  	td->td_kstack = NULL;
  	td->td_kstack_size = 0;
      }
      KTR_LOG(ctxsw_deadtd, td);
  }
  
  
  /*
   * Switch to the next runnable lwkt.  If no LWKTs are runnable then 
   * switch to the idlethread.  Switching must occur within a critical
   * section to avoid races with the scheduling queue.
   *
   * We always have full control over our cpu's run queue.  Other cpus
   * that wish to manipulate our queue must use the cpu_*msg() calls to
   * talk to our cpu, so a critical section is all that is needed and
   * the result is very, very fast thread switching.
   *
   * The LWKT scheduler uses a fixed priority model and round-robins at
   * each priority level.  User process scheduling is a totally
   * different beast and LWKT priorities should not be confused with
   * user process priorities.
   *
   * PREEMPTION NOTE: Preemption occurs via lwkt_preempt().  lwkt_switch()
   * is not called by the current thread in the preemption case, only when
   * the preempting thread blocks (in order to return to the original thread).
   *
   * SPECIAL NOTE ON SWITCH ATOMICY: Certain operations such as thread
   * migration and tsleep deschedule the current lwkt thread and call
   * lwkt_switch().  In particular, the target cpu of the migration fully
   * expects the thread to become non-runnable and can deadlock against
   * cpusync operations if we run any IPIs prior to switching the thread out.
   *
   * WE MUST BE VERY CAREFUL NOT TO RUN SPLZ DIRECTLY OR INDIRECTLY IF
   * THE CURRENT THREAD HAS BEEN DESCHEDULED!
   */
  void
  lwkt_switch(void)
  {
      globaldata_t gd = mycpu;
      thread_t td = gd->gd_curthread;
      thread_t ntd;
      int spinning = 0;
  
      KKASSERT(gd->gd_processing_ipiq == 0);
      KKASSERT(td->td_flags & TDF_RUNNING);
  
      /*
       * Switching from within a 'fast' (non thread switched) interrupt or IPI
       * is illegal.  However, we may have to do it anyway if we hit a fatal
       * kernel trap or we have paniced.
       *
       * If this case occurs save and restore the interrupt nesting level.
       */
      if (gd->gd_intr_nesting_level) {
  	int savegdnest;
  	int savegdtrap;
  
  	if (gd->gd_trap_nesting_level == 0 && panic_cpu_gd != mycpu) {
  	    panic("lwkt_switch: Attempt to switch from a "
  		  "fast interrupt, ipi, or hard code section, "
  		  "td %p\n",
  		  td);
  	} else {
  	    savegdnest = gd->gd_intr_nesting_level;
  	    savegdtrap = gd->gd_trap_nesting_level;
  	    gd->gd_intr_nesting_level = 0;
  	    gd->gd_trap_nesting_level = 0;
  	    if ((td->td_flags & TDF_PANICWARN) == 0) {
  		td->td_flags |= TDF_PANICWARN;
  		kprintf("Warning: thread switch from interrupt, IPI, "
  			"or hard code section.\n"
  			"thread %p (%s)\n", td, td->td_comm);
  		print_backtrace(-1);
  	    }
  	    lwkt_switch();
  	    gd->gd_intr_nesting_level = savegdnest;
  	    gd->gd_trap_nesting_level = savegdtrap;
  	    return;
  	}
      }
  
      /*
       * Release our current user process designation if we are blocking
       * or if a user reschedule was requested.
       *
       * NOTE: This function is NOT called if we are switching into or
       *	     returning from a preemption.
       *
       * NOTE: Releasing our current user process designation may cause
       *	     it to be assigned to another thread, which in turn will
       *	     cause us to block in the usched acquire code when we attempt
       *	     to return to userland.
       *
       * NOTE: On SMP systems this can be very nasty when heavy token
       *	     contention is present so we want to be careful not to
       *	     release the designation gratuitously.
       */
      if (td->td_release &&
  	(user_resched_wanted() || (td->td_flags & TDF_RUNQ) == 0)) {
  	    td->td_release(td);
      }
  
      /*
       * Release all tokens
       */
      crit_enter_gd(gd);
      if (TD_TOKS_HELD(td))
  	    lwkt_relalltokens(td);
  
      /*
       * We had better not be holding any spin locks, but don't get into an
       * endless panic loop.
       */
      KASSERT(gd->gd_spinlocks == 0 || panicstr != NULL,
  	    ("lwkt_switch: still holding %d exclusive spinlocks!",
  	     gd->gd_spinlocks));
  
  
  #ifdef	INVARIANTS
      if (td->td_cscount) {
  	kprintf("Diagnostic: attempt to switch while mastering cpusync: %p\n",
  		td);
  	if (panic_on_cscount)
  	    panic("switching while mastering cpusync");
      }
  #endif
  
      /*
       * If we had preempted another thread on this cpu, resume the preempted
       * thread.  This occurs transparently, whether the preempted thread
       * was scheduled or not (it may have been preempted after descheduling
       * itself).
       *
       * We have to setup the MP lock for the original thread after backing
       * out the adjustment that was made to curthread when the original
       * was preempted.
       */
      if ((ntd = td->td_preempted) != NULL) {
  	KKASSERT(ntd->td_flags & TDF_PREEMPT_LOCK);
  	ntd->td_flags |= TDF_PREEMPT_DONE;
  
  	/*
  	 * The interrupt may have woken a thread up, we need to properly
  	 * set the reschedule flag if the originally interrupted thread is
  	 * at a lower priority.
  	 *
  	 * The interrupt may not have descheduled.
  	 */
  	if (TAILQ_FIRST(&gd->gd_tdrunq) != ntd)
  	    need_lwkt_resched();
  	goto havethread_preempted;
      }
  
      /*
       * If we cannot obtain ownership of the tokens we cannot immediately
       * schedule the target thread.
       *
       * Reminder: Again, we cannot afford to run any IPIs in this path if
       * the current thread has been descheduled.
       */
      for (;;) {
  	clear_lwkt_resched();
  
  	/*
  	 * Hotpath - pull the head of the run queue and attempt to schedule
  	 * it.
  	 */
  	ntd = TAILQ_FIRST(&gd->gd_tdrunq);
  
  	if (ntd == NULL) {
  	    /*
  	     * Runq is empty, switch to idle to allow it to halt.
  	     */
  	    ntd = &gd->gd_idlethread;
  	    if (gd->gd_trap_nesting_level == 0 && panicstr == NULL)
  		ASSERT_NO_TOKENS_HELD(ntd);
  	    cpu_time.cp_msg[0] = 0;
  	    cpu_time.cp_stallpc = 0;
  	    goto haveidle;
  	}
  
  	/*
  	 * Hotpath - schedule ntd.
  	 *
  	 * NOTE: For UP there is no mplock and lwkt_getalltokens()
  	 *	     always succeeds.
  	 */
  	if (TD_TOKS_NOT_HELD(ntd) ||
  	    lwkt_getalltokens(ntd, (spinning >= lwkt_spin_loops)))
  	{
  	    goto havethread;
  	}
  
  	/*
  	 * Coldpath (SMP only since tokens always succeed on UP)
  	 *
  	 * We had some contention on the thread we wanted to schedule.
  	 * What we do now is try to find a thread that we can schedule
  	 * in its stead.
  	 *
  	 * The coldpath scan does NOT rearrange threads in the run list.
  	 * The lwkt_schedulerclock() will assert need_lwkt_resched() on
  	 * the next tick whenever the current head is not the current thread.
  	 */
  #ifdef	INVARIANTS
  	++ntd->td_contended;
  #endif
  	++gd->gd_cnt.v_token_colls;
  
  	if (fairq_bypass > 0)
  		goto skip;
  
  	while ((ntd = TAILQ_NEXT(ntd, td_threadq)) != NULL) {
  #ifndef NO_LWKT_SPLIT_USERPRI
  		/*
  		 * Never schedule threads returning to userland or the
  		 * user thread scheduler helper thread when higher priority
  		 * threads are present.  The runq is sorted by priority
  		 * so we can give up traversing it when we find the first
  		 * low priority thread.
  		 */
  		if (ntd->td_pri < TDPRI_KERN_LPSCHED) {
  			ntd = NULL;
  			break;
  		}
  #endif
  
  		/*
  		 * Try this one.
  		 */
  		if (TD_TOKS_NOT_HELD(ntd) ||
  		    lwkt_getalltokens(ntd, (spinning >= lwkt_spin_loops))) {
  			goto havethread;
  		}
  #ifdef	INVARIANTS
  		++ntd->td_contended;
  #endif
  		++gd->gd_cnt.v_token_colls;
  	}
  
  skip:
  	/*
  	 * We exhausted the run list, meaning that all runnable threads
  	 * are contested.
  	 */
  	cpu_pause();
  	ntd = &gd->gd_idlethread;
  	if (gd->gd_trap_nesting_level == 0 && panicstr == NULL)
  	    ASSERT_NO_TOKENS_HELD(ntd);
  	/* contention case, do not clear contention mask */
  
  	/*
  	 * We are going to have to retry but if the current thread is not
  	 * on the runq we instead switch through the idle thread to get away
  	 * from the current thread.  We have to flag for lwkt reschedule
  	 * to prevent the idle thread from halting.
  	 *
  	 * NOTE: A non-zero spinning is passed to lwkt_getalltokens() to
  	 *	 instruct it to deal with the potential for deadlocks by
  	 *	 ordering the tokens by address.
  	 */
  	if ((td->td_flags & TDF_RUNQ) == 0) {
  	    need_lwkt_resched();	/* prevent hlt */
  	    goto haveidle;
  	}
  #if defined(INVARIANTS) && defined(__x86_64__)
  	if ((read_rflags() & PSL_I) == 0) {
  		cpu_enable_intr();
  		panic("lwkt_switch() called with interrupts disabled");
  	}
  #endif
  
  	/*
  	 * Number iterations so far.  After a certain point we switch to
  	 * a sorted-address/monitor/mwait version of lwkt_getalltokens()
  	 */
  	if (spinning < 0x7FFFFFFF)
  	    ++spinning;
  
  	/*
  	 * lwkt_getalltokens() failed in sorted token mode, we can use
  	 * monitor/mwait in this case.
  	 */
  	if (spinning >= lwkt_spin_loops &&
  	    (cpu_mi_feature & CPU_MI_MONITOR) &&
  	    lwkt_spin_monitor)
  	{
  	    cpu_mmw_pause_int(&gd->gd_reqflags,
  			      (gd->gd_reqflags | RQF_SPINNING) &
  			      ~RQF_IDLECHECK_WK_MASK);
  	}
  
  	/*
  	 * We already checked that td is still scheduled so this should be
  	 * safe.
  	 */
  	splz_check();
  
  	/*
  	 * This experimental resequencer is used as a fall-back to reduce
  	 * hw cache line contention by placing each core's scheduler into a
  	 * time-domain-multplexed slot.
  	 *
  	 * The resequencer is disabled by default.  It's functionality has
  	 * largely been superceeded by the token algorithm which limits races
  	 * to a subset of cores.
  	 *
  	 * The resequencer algorithm tends to break down when more than
  	 * 20 cores are contending.  What appears to happen is that new
  	 * tokens can be obtained out of address-sorted order by new cores
  	 * while existing cores languish in long delays between retries and
  	 * wind up being starved-out of the token acquisition.
  	 */
  	if (lwkt_spin_reseq && spinning >= lwkt_spin_reseq) {
  	    int cseq = atomic_fetchadd_int(&lwkt_cseq_windex, 1);
  	    int oseq;
  
  	    while ((oseq = lwkt_cseq_rindex) != cseq) {
  		cpu_ccfence();
  #if 1
  		if (cpu_mi_feature & CPU_MI_MONITOR) {
  		    cpu_mmw_pause_int(&lwkt_cseq_rindex, oseq);
  		} else {
  #endif
  		    cpu_pause();
  		    cpu_lfence();
  #if 1
  		}
  #endif
  	    }
  	    DELAY(1);
  	    atomic_add_int(&lwkt_cseq_rindex, 1);
  	}
  	/* highest level for(;;) loop */
      }
  
  havethread:
      /*
       * Clear gd_idle_repeat when doing a normal switch to a non-idle
       * thread.
       */
      ntd->td_wmesg = NULL;
      ++gd->gd_cnt.v_swtch;
      gd->gd_idle_repeat = 0;
  
  havethread_preempted:
      /*
       * If the new target does not need the MP lock and we are holding it,
       * release the MP lock.  If the new target requires the MP lock we have
       * already acquired it for the target.
       */
      ;
  haveidle:
      KASSERT(ntd->td_critcount,
  	    ("priority problem in lwkt_switch %d %d",
  	    td->td_critcount, ntd->td_critcount));
  
      if (td != ntd) {
  	/*
  	 * Execute the actual thread switch operation.  This function
  	 * returns to the current thread and returns the previous thread
  	 * (which may be different from the thread we switched to).
  	 *
  	 * We are responsible for marking ntd as TDF_RUNNING.
  	 */
  	KKASSERT((ntd->td_flags & TDF_RUNNING) == 0);
  	++switch_count;
  	KTR_LOG(ctxsw_sw, gd->gd_cpuid, ntd);
  	ntd->td_flags |= TDF_RUNNING;
  	lwkt_switch_return(td->td_switch(ntd));
  	/* ntd invalid, td_switch() can return a different thread_t */
      }
  
      /*
       * catch-all.  XXX is this strictly needed?
       */
      splz_check();
  
      /* NOTE: current cpu may have changed after switch */
      crit_exit_quick(td);
  }
  
  /*
   * Called by assembly in the td_switch (thread restore path) for thread
   * bootstrap cases which do not 'return' to lwkt_switch().
   */
  void
  lwkt_switch_return(thread_t otd)
  {
  	globaldata_t rgd;
  
  	/*
  	 * Check if otd was migrating.  Now that we are on ntd we can finish
  	 * up the migration.  This is a bit messy but it is the only place
  	 * where td is known to be fully descheduled.
  	 *
  	 * We can only activate the migration if otd was migrating but not
  	 * held on the cpu due to a preemption chain.  We still have to
  	 * clear TDF_RUNNING on the old thread either way.
  	 *
  	 * We are responsible for clearing the previously running thread's
  	 * TDF_RUNNING.
  	 */
  	if ((rgd = otd->td_migrate_gd) != NULL &&
  	    (otd->td_flags & TDF_PREEMPT_LOCK) == 0) {
  		KKASSERT((otd->td_flags & (TDF_MIGRATING | TDF_RUNNING)) ==
  			 (TDF_MIGRATING | TDF_RUNNING));
  		otd->td_migrate_gd = NULL;
  		otd->td_flags &= ~TDF_RUNNING;
  		lwkt_send_ipiq(rgd, lwkt_setcpu_remote, otd);
  	} else {
  		otd->td_flags &= ~TDF_RUNNING;
  	}
  
  	/*
  	 * Final exit validations (see lwp_wait()).  Note that otd becomes
  	 * invalid the *instant* we set TDF_MP_EXITSIG.
  	 */
  	while (otd->td_flags & TDF_EXITING) {
  		u_int mpflags;
  
  		mpflags = otd->td_mpflags;
  		cpu_ccfence();
  
  		if (mpflags & TDF_MP_EXITWAIT) {
  			if (atomic_cmpset_int(&otd->td_mpflags, mpflags,
  					      mpflags | TDF_MP_EXITSIG)) {
  				wakeup(otd);
  				break;
  			}
  		} else {
  			if (atomic_cmpset_int(&otd->td_mpflags, mpflags,
  					      mpflags | TDF_MP_EXITSIG)) {
  				wakeup(otd);
  				break;
  			}
  		}
  	}
  }
  
  /*
   * Request that the target thread preempt the current thread.  Preemption
   * can only occur if our only critical section is the one that we were called
   * with, the relative priority of the target thread is higher, and the target
   * thread holds no tokens.  This also only works if we are not holding any
   * spinlocks (obviously).
   *
   * THE CALLER OF LWKT_PREEMPT() MUST BE IN A CRITICAL SECTION.  Typically
   * this is called via lwkt_schedule() through the td_preemptable callback.
   * critcount is the managed critical priority that we should ignore in order
   * to determine whether preemption is possible (aka usually just the crit
   * priority of lwkt_schedule() itself).
   *
   * Preemption is typically limited to interrupt threads.
   *
   * Operation works in a fairly straight-forward manner.  The normal
   * scheduling code is bypassed and we switch directly to the target
   * thread.  When the target thread attempts to block or switch away
   * code at the base of lwkt_switch() will switch directly back to our
   * thread.  Our thread is able to retain whatever tokens it holds and
   * if the target needs one of them the target will switch back to us
   * and reschedule itself normally.
   */
  void
  lwkt_preempt(thread_t ntd, int critcount)
  {
      struct globaldata *gd = mycpu;
      thread_t xtd;
      thread_t td;
      int save_gd_intr_nesting_level;
  
      /*
       * The caller has put us in a critical section.  We can only preempt
       * if the caller of the caller was not in a critical section (basically
       * a local interrupt), as determined by the 'critcount' parameter.  We
       * also can't preempt if the caller is holding any spinlocks (even if
       * he isn't in a critical section).  This also handles the tokens test.
       *
       * YYY The target thread must be in a critical section (else it must
       * inherit our critical section?  I dunno yet).
       */
      KASSERT(ntd->td_critcount, ("BADCRIT0 %d", ntd->td_pri));
  
      td = gd->gd_curthread;
      if (preempt_enable == 0) {
  	++preempt_miss;
  	return;
      }
      if (ntd->td_pri <= td->td_pri) {
  	++preempt_miss;
  	return;
      }
      if (td->td_critcount > critcount) {
  	++preempt_miss;
  	return;
      }
      if (td->td_cscount) {
  	++preempt_miss;
  	return;
      }
      if (ntd->td_gd != gd) {
  	++preempt_miss;
  	return;
      }
      /*
       * We don't have to check spinlocks here as they will also bump
       * td_critcount.
       *
       * Do not try to preempt if the target thread is holding any tokens.
       * We could try to acquire the tokens but this case is so rare there
       * is no need to support it.
       */
      KKASSERT(gd->gd_spinlocks == 0);
  
      if (TD_TOKS_HELD(ntd)) {
  	++preempt_miss;
  	return;
      }
      if (td == ntd || ((td->td_flags | ntd->td_flags) & TDF_PREEMPT_LOCK)) {
  	++preempt_weird;
  	return;
      }
      if (ntd->td_preempted) {
  	++preempt_hit;
  	return;
      }
      KKASSERT(gd->gd_processing_ipiq == 0);
  
      /*
       * Since we are able to preempt the current thread, there is no need to
       * call need_lwkt_resched().
       *
       * We must temporarily clear gd_intr_nesting_level around the switch
       * since switchouts from the target thread are allowed (they will just
       * return to our thread), and since the target thread has its own stack.
       *
       * A preemption must switch back to the original thread, assert the
       * case.
       */
      ++preempt_hit;
      ntd->td_preempted = td;
      td->td_flags |= TDF_PREEMPT_LOCK;
      KTR_LOG(ctxsw_pre, gd->gd_cpuid, ntd);
      save_gd_intr_nesting_level = gd->gd_intr_nesting_level;
      gd->gd_intr_nesting_level = 0;
  
      KKASSERT((ntd->td_flags & TDF_RUNNING) == 0);
      ntd->td_flags |= TDF_RUNNING;
      xtd = td->td_switch(ntd);
      KKASSERT(xtd == ntd);
      lwkt_switch_return(xtd);
      gd->gd_intr_nesting_level = save_gd_intr_nesting_level;
  
      KKASSERT(ntd->td_preempted && (td->td_flags & TDF_PREEMPT_DONE));
      ntd->td_preempted = NULL;
      td->td_flags &= ~(TDF_PREEMPT_LOCK|TDF_PREEMPT_DONE);
  }
  
  /*
   * Conditionally call splz() if gd_reqflags indicates work is pending.
   * This will work inside a critical section but not inside a hard code
   * section.
   *
   * (self contained on a per cpu basis)
   */
  void
  splz_check(void)
  {
      globaldata_t gd = mycpu;
      thread_t td = gd->gd_curthread;
  
      if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) &&
  	gd->gd_intr_nesting_level == 0 &&
  	td->td_nest_count < 2)
      {
  	splz();
      }
  }
  
  /*
   * This version is integrated into crit_exit, reqflags has already
   * been tested but td_critcount has not.
   *
   * We only want to execute the splz() on the 1->0 transition of
   * critcount and not in a hard code section or if too deeply nested.
   *
   * NOTE: gd->gd_spinlocks is implied to be 0 when td_critcount is 0.
   */
  void
  lwkt_maybe_splz(thread_t td)
  {
      globaldata_t gd = td->td_gd;
  
      if (td->td_critcount == 0 &&
  	gd->gd_intr_nesting_level == 0 &&
  	td->td_nest_count < 2)
      {
  	splz();
      }
  }
  
  /*
   * Drivers which set up processing co-threads can call this function to
   * run the co-thread at a higher priority and to allow it to preempt
   * normal threads.
   */
  void
  lwkt_set_interrupt_support_thread(void)
  {
  	thread_t td = curthread;
  
          lwkt_setpri_self(TDPRI_INT_SUPPORT);
  	td->td_flags |= TDF_INTTHREAD;
  	td->td_preemptable = lwkt_preempt;
  }
  
  
  /*
   * This function is used to negotiate a passive release of the current
   * process/lwp designation with the user scheduler, allowing the user
   * scheduler to schedule another user thread.  The related kernel thread
   * (curthread) continues running in the released state.
   */
  void
  lwkt_passive_release(struct thread *td)
  {
      struct lwp *lp = td->td_lwp;
  
  #ifndef NO_LWKT_SPLIT_USERPRI
      td->td_release = NULL;
      lwkt_setpri_self(TDPRI_KERN_USER);
  #endif
  
      lp->lwp_proc->p_usched->release_curproc(lp);
  }
  
  
  /*
   * This implements a LWKT yield, allowing a kernel thread to yield to other
   * kernel threads at the same or higher priority.  This function can be
   * called in a tight loop and will typically only yield once per tick.
   *
   * Most kernel threads run at the same priority in order to allow equal
   * sharing.
   *
   * (self contained on a per cpu basis)
   */
  void
  lwkt_yield(void)
  {
      globaldata_t gd = mycpu;
      thread_t td = gd->gd_curthread;
  
      if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) && td->td_nest_count < 2)
  	splz();
      if (lwkt_resched_wanted()) {
  	lwkt_schedule_self(curthread);
  	lwkt_switch();
      }
  }
  
  /*
   * The quick version processes pending interrupts and higher-priority
   * LWKT threads but will not round-robin same-priority LWKT threads.
   *
   * When called while attempting to return to userland the only same-pri
   * threads are the ones which have already tried to become the current
   * user process.
   */
  void
  lwkt_yield_quick(void)
  {
      globaldata_t gd = mycpu;
      thread_t td = gd->gd_curthread;
  
      if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) && td->td_nest_count < 2)
  	splz();
      if (lwkt_resched_wanted()) {
  	if (TAILQ_FIRST(&gd->gd_tdrunq) == td) {
  	    clear_lwkt_resched();
  	} else {
  	    lwkt_schedule_self(curthread);
  	    lwkt_switch();
  	}
      }
  }
  
  /*
   * This yield is designed for kernel threads with a user context.
   *
   * The kernel acting on behalf of the user is potentially cpu-bound,
   * this function will efficiently allow other threads to run and also
   * switch to other processes by releasing.
   *
   * The lwkt_user_yield() function is designed to have very low overhead
   * if no yield is determined to be needed.
   */
  void
  lwkt_user_yield(void)
  {
      globaldata_t gd = mycpu;
      thread_t td = gd->gd_curthread;
  
      /*
       * Always run any pending interrupts in case we are in a critical
       * section.
       */
      if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) && td->td_nest_count < 2)
  	splz();
  
      /*
       * Switch (which forces a release) if another kernel thread needs
       * the cpu, if userland wants us to resched, or if our kernel
       * quantum has run out.
       */
      if (lwkt_resched_wanted() ||
  	user_resched_wanted())
      {
  	lwkt_switch();
      }
  
  #if 0
      /*
       * Reacquire the current process if we are released.
       *
       * XXX not implemented atm.  The kernel may be holding locks and such,
       *     so we want the thread to continue to receive cpu.
       */
      if (td->td_release == NULL && lp) {
  	lp->lwp_proc->p_usched->acquire_curproc(lp);
  	td->td_release = lwkt_passive_release;
  	lwkt_setpri_self(TDPRI_USER_NORM);
      }
  #endif
  }
  
  /*
   * Generic schedule.  Possibly schedule threads belonging to other cpus and
   * deal with threads that might be blocked on a wait queue.
   *
   * We have a little helper inline function which does additional work after
   * the thread has been enqueued, including dealing with preemption and
   * setting need_lwkt_resched() (which prevents the kernel from returning
   * to userland until it has processed higher priority threads).
   *
   * It is possible for this routine to be called after a failed _enqueue
   * (due to the target thread migrating, sleeping, or otherwise blocked).
   * We have to check that the thread is actually on the run queue!
   */
  static __inline
  void
  _lwkt_schedule_post(globaldata_t gd, thread_t ntd, int ccount)
  {
      if (ntd->td_flags & TDF_RUNQ) {
  	if (ntd->td_preemptable) {
  	    ntd->td_preemptable(ntd, ccount);	/* YYY +token */
  	}
      }
  }
  
  static __inline
  void
  _lwkt_schedule(thread_t td)
  {
      globaldata_t mygd = mycpu;
  
      KASSERT(td != &td->td_gd->gd_idlethread,
  	    ("lwkt_schedule(): scheduling gd_idlethread is illegal!"));
      KKASSERT((td->td_flags & TDF_MIGRATING) == 0);
      crit_enter_gd(mygd);
      KKASSERT(td->td_lwp == NULL ||
  	     (td->td_lwp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
  
      if (td == mygd->gd_curthread) {
  	_lwkt_enqueue(td);
      } else {
  	/*
  	 * If we own the thread, there is no race (since we are in a
  	 * critical section).  If we do not own the thread there might
  	 * be a race but the target cpu will deal with it.
  	 */
  	if (td->td_gd == mygd) {
  	    _lwkt_enqueue(td);
  	    _lwkt_schedule_post(mygd, td, 1);
  	} else {
  	    lwkt_send_ipiq3(td->td_gd, lwkt_schedule_remote, td, 0);
  	}
      }
      crit_exit_gd(mygd);
  }
  
  void
  lwkt_schedule(thread_t td)
  {
      _lwkt_schedule(td);
  }
  
  void
  lwkt_schedule_noresched(thread_t td)	/* XXX not impl */
  {
      _lwkt_schedule(td);
  }
  
  /*
   * When scheduled remotely if frame != NULL the IPIQ is being
   * run via doreti or an interrupt then preemption can be allowed.
   *
   * To allow preemption we have to drop the critical section so only
   * one is present in _lwkt_schedule_post.
   */
  static void
  lwkt_schedule_remote(void *arg, int arg2, struct intrframe *frame)
  {
      thread_t td = curthread;
      thread_t ntd = arg;
  
      if (frame && ntd->td_preemptable) {
  	crit_exit_noyield(td);
  	_lwkt_schedule(ntd);
  	crit_enter_quick(td);
      } else {
  	_lwkt_schedule(ntd);
      }
  }
  
  /*
   * Thread migration using a 'Pull' method.  The thread may or may not be
   * the current thread.  It MUST be descheduled and in a stable state.
   * lwkt_giveaway() must be called on the cpu owning the thread.
   *
   * At any point after lwkt_giveaway() is called, the target cpu may
   * 'pull' the thread by calling lwkt_acquire().
   *
   * We have to make sure the thread is not sitting on a per-cpu tsleep
   * queue or it will blow up when it moves to another cpu.
   *
   * MPSAFE - must be called under very specific conditions.
   */
  void
  lwkt_giveaway(thread_t td)
  {
      globaldata_t gd = mycpu;
  
      crit_enter_gd(gd);
      if (td->td_flags & TDF_TSLEEPQ)
  	tsleep_remove(td);
      KKASSERT(td->td_gd == gd);
      TAILQ_REMOVE(&gd->gd_tdallq, td, td_allq);
      td->td_flags |= TDF_MIGRATING;
      crit_exit_gd(gd);
  }
  
  void
  lwkt_acquire(thread_t td)
  {
      globaldata_t gd;
      globaldata_t mygd;
      int retry = 10000000;
  
      KKASSERT(td->td_flags & TDF_MIGRATING);
      gd = td->td_gd;
      mygd = mycpu;
      if (gd != mycpu) {
  	cpu_lfence();
  	KKASSERT((td->td_flags & TDF_RUNQ) == 0);
  	crit_enter_gd(mygd);
  	DEBUG_PUSH_INFO("lwkt_acquire");
  	while (td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) {
  	    lwkt_process_ipiq();
  	    cpu_lfence();
  	    if (--retry == 0) {
  		kprintf("lwkt_acquire: stuck: td %p td->td_flags %08x\n",
  			td, td->td_flags);
  		retry = 10000000;
  	    }
  	}
  	DEBUG_POP_INFO();
  	cpu_mfence();
  	td->td_gd = mygd;
  	TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
  	td->td_flags &= ~TDF_MIGRATING;
  	crit_exit_gd(mygd);
      } else {
  	crit_enter_gd(mygd);
  	TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
  	td->td_flags &= ~TDF_MIGRATING;
  	crit_exit_gd(mygd);
      }
  }
  
  /*
   * Generic deschedule.  Descheduling threads other then your own should be
   * done only in carefully controlled circumstances.  Descheduling is 
   * asynchronous.  
   *
   * This function may block if the cpu has run out of messages.
   */
  void
  lwkt_deschedule(thread_t td)
  {
      crit_enter();
      if (td == curthread) {
  	_lwkt_dequeue(td);
      } else {
  	if (td->td_gd == mycpu) {
  	    _lwkt_dequeue(td);
  	} else {
  	    lwkt_send_ipiq(td->td_gd, (ipifunc1_t)lwkt_deschedule, td);
  	}
      }
      crit_exit();
  }
  
  /*
   * Set the target thread's priority.  This routine does not automatically
   * switch to a higher priority thread, LWKT threads are not designed for
   * continuous priority changes.  Yield if you want to switch.
   */
  void
  lwkt_setpri(thread_t td, int pri)
  {
      if (td->td_pri != pri) {
  	KKASSERT(pri >= 0);
  	crit_enter();
  	if (td->td_flags & TDF_RUNQ) {
  	    KKASSERT(td->td_gd == mycpu);
  	    _lwkt_dequeue(td);
  	    td->td_pri = pri;
  	    _lwkt_enqueue(td);
  	} else {
  	    td->td_pri = pri;
  	}
  	crit_exit();
      }
  }
  
  /*
   * Set the initial priority for a thread prior to it being scheduled for
   * the first time.  The thread MUST NOT be scheduled before or during
   * this call.  The thread may be assigned to a cpu other then the current
   * cpu.
   *
   * Typically used after a thread has been created with TDF_STOPPREQ,
   * and before the thread is initially scheduled.
   */
  void
  lwkt_setpri_initial(thread_t td, int pri)
  {
      KKASSERT(pri >= 0);
      KKASSERT((td->td_flags & TDF_RUNQ) == 0);
      td->td_pri = pri;
  }
  
  void
  lwkt_setpri_self(int pri)
  {
      thread_t td = curthread;
  
      KKASSERT(pri >= 0 && pri <= TDPRI_MAX);
      crit_enter();
      if (td->td_flags & TDF_RUNQ) {
  	_lwkt_dequeue(td);
  	td->td_pri = pri;
  	_lwkt_enqueue(td);
      } else {
  	td->td_pri = pri;
      }
      crit_exit();
  }
  
  /*
   * hz tick scheduler clock for LWKT threads
   */
  void
  lwkt_schedulerclock(thread_t td)
  {
      globaldata_t gd = td->td_gd;
      thread_t xtd;
  
      if (TAILQ_FIRST(&gd->gd_tdrunq) == td) {
  	/*
  	 * If the current thread is at the head of the runq shift it to the
  	 * end of any equal-priority threads and request a LWKT reschedule
  	 * if it moved.
  	 *
  	 * Ignore upri in this situation.  There will only be one user thread
  	 * in user mode, all others will be user threads running in kernel
  	 * mode and we have to make sure they get some cpu.
  	 */
  	xtd = TAILQ_NEXT(td, td_threadq);
  	if (xtd && xtd->td_pri == td->td_pri) {
  	    TAILQ_REMOVE(&gd->gd_tdrunq, td, td_threadq);
  	    while (xtd && xtd->td_pri == td->td_pri)
  		xtd = TAILQ_NEXT(xtd, td_threadq);
  	    if (xtd)
  		TAILQ_INSERT_BEFORE(xtd, td, td_threadq);
  	    else
  		TAILQ_INSERT_TAIL(&gd->gd_tdrunq, td, td_threadq);
  	    need_lwkt_resched();
  	}
      } else {
  	/*
  	 * If we scheduled a thread other than the one at the head of the
  	 * queue always request a reschedule every tick.
  	 */
  	need_lwkt_resched();
      }
  }
  
  /*
   * Migrate the current thread to the specified cpu. 
   *
   * This is accomplished by descheduling ourselves from the current cpu
   * and setting td_migrate_gd.  The lwkt_switch() code will detect that the
   * 'old' thread wants to migrate after it has been completely switched out
   * and will complete the migration.
   *
   * TDF_MIGRATING prevents scheduling races while the thread is being migrated.
   *
   * We must be sure to release our current process designation (if a user
   * process) before clearing out any tsleepq we are on because the release
   * code may re-add us.
   *
   * We must be sure to remove ourselves from the current cpu's tsleepq
   * before potentially moving to another queue.  The thread can be on
   * a tsleepq due to a left-over tsleep_interlock().
   */
  
  void
  lwkt_setcpu_self(globaldata_t rgd)
  {
      thread_t td = curthread;
  
      if (td->td_gd != rgd) {
  	crit_enter_quick(td);
  
  	if (td->td_release)
  	    td->td_release(td);
  	if (td->td_flags & TDF_TSLEEPQ)
  	    tsleep_remove(td);
  
  	/*
  	 * Set TDF_MIGRATING to prevent a spurious reschedule while we are
  	 * trying to deschedule ourselves and switch away, then deschedule
  	 * ourself, remove us from tdallq, and set td_migrate_gd.  Finally,
  	 * call lwkt_switch() to complete the operation.
  	 */
  	td->td_flags |= TDF_MIGRATING;
  	lwkt_deschedule_self(td);
  	TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
  	td->td_migrate_gd = rgd;
  	lwkt_switch();
  
  	/*
  	 * We are now on the target cpu
  	 */
  	KKASSERT(rgd == mycpu);
  	TAILQ_INSERT_TAIL(&rgd->gd_tdallq, td, td_allq);
  	crit_exit_quick(td);
      }
  }
  
  void
  lwkt_migratecpu(int cpuid)
  {
  	globaldata_t rgd;
  
  	rgd = globaldata_find(cpuid);
  	lwkt_setcpu_self(rgd);
  }
  
  /*
   * Remote IPI for cpu migration (called while in a critical section so we
   * do not have to enter another one).
   *
   * The thread (td) has already been completely descheduled from the
   * originating cpu and we can simply assert the case.  The thread is
   * assigned to the new cpu and enqueued.
   *
   * The thread will re-add itself to tdallq when it resumes execution.
   */
  static void
  lwkt_setcpu_remote(void *arg)
  {
      thread_t td = arg;
      globaldata_t gd = mycpu;
  
      KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) == 0);
      td->td_gd = gd;
      cpu_mfence();
      td->td_flags &= ~TDF_MIGRATING;
      KKASSERT(td->td_migrate_gd == NULL);
      KKASSERT(td->td_lwp == NULL ||
  	    (td->td_lwp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
      _lwkt_enqueue(td);
  }
  
  struct lwp *
  lwkt_preempted_proc(void)
  {
      thread_t td = curthread;
      while (td->td_preempted)
  	td = td->td_preempted;
      return(td->td_lwp);
  }
  
  /*
   * Create a kernel process/thread/whatever.  It shares it's address space
   * with proc0 - ie: kernel only.
   *
   * If the cpu is not specified one will be selected.  In the future
   * specifying a cpu of -1 will enable kernel thread migration between
   * cpus.
   */
  int
  lwkt_create(void (*func)(void *), void *arg, struct thread **tdp,
  	    thread_t template, int tdflags, int cpu, const char *fmt, ...)
  {
      thread_t td;
      __va_list ap;
  
      td = lwkt_alloc_thread(template, LWKT_THREAD_STACK, cpu,
  			   tdflags);
      if (tdp)
  	*tdp = td;
      cpu_set_thread_handler(td, lwkt_exit, func, arg);
  
      /*
       * Set up arg0 for 'ps' etc
       */
      __va_start(ap, fmt);
      kvsnprintf(td->td_comm, sizeof(td->td_comm), fmt, ap);
      __va_end(ap);
  
      /*
       * Schedule the thread to run
       */
      if (td->td_flags & TDF_NOSTART)
  	td->td_flags &= ~TDF_NOSTART;
      else
  	lwkt_schedule(td);
      return 0;
  }
  
  /*
   * Destroy an LWKT thread.   Warning!  This function is not called when
   * a process exits, cpu_proc_exit() directly calls cpu_thread_exit() and
   * uses a different reaping mechanism.
   */
  void
  lwkt_exit(void)
  {
      thread_t td = curthread;
      thread_t std;
      globaldata_t gd;
  
      /*
       * Do any cleanup that might block here
       */
      if (td->td_flags & TDF_VERBOSE)
  	kprintf("kthread %p %s has exited\n", td, td->td_comm);
      biosched_done(td);
      dsched_exit_thread(td);
  
      /*
       * Get us into a critical section to interlock gd_freetd and loop
       * until we can get it freed.
       *
       * We have to cache the current td in gd_freetd because objcache_put()ing
       * it would rip it out from under us while our thread is still active.
       *
       * We are the current thread so of course our own TDF_RUNNING bit will
       * be set, so unlike the lwp reap code we don't wait for it to clear.
       */
      gd = mycpu;
      crit_enter_quick(td);
      for (;;) {
  	if (td->td_refs) {
  	    tsleep(td, 0, "tdreap", 1);
  	    continue;
  	}
  	if ((std = gd->gd_freetd) != NULL) {
  	    KKASSERT((std->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) == 0);
  	    gd->gd_freetd = NULL;
  	    objcache_put(thread_cache, std);
  	    continue;
  	}
  	break;
      }
  
      /*
       * Remove thread resources from kernel lists and deschedule us for
       * the last time.  We cannot block after this point or we may end
       * up with a stale td on the tsleepq.
       *
       * None of this may block, the critical section is the only thing
       * protecting tdallq and the only thing preventing new lwkt_hold()
       * thread refs now.
       */
      if (td->td_flags & TDF_TSLEEPQ)
  	tsleep_remove(td);
      lwkt_deschedule_self(td);
      lwkt_remove_tdallq(td);
      KKASSERT(td->td_refs == 0);
  
      /*
       * Final cleanup
       */
      KKASSERT(gd->gd_freetd == NULL);
      if (td->td_flags & TDF_ALLOCATED_THREAD)
  	gd->gd_freetd = td;
      cpu_thread_exit();
  }
  
  void
  lwkt_remove_tdallq(thread_t td)
  {
      KKASSERT(td->td_gd == mycpu);
      TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
  }
  
  /*
   * Code reduction and branch prediction improvements.  Call/return
   * overhead on modern cpus often degenerates into 0 cycles due to
   * the cpu's branch prediction hardware and return pc cache.  We
   * can take advantage of this by not inlining medium-complexity
   * functions and we can also reduce the branch prediction impact
   * by collapsing perfectly predictable branches into a single
   * procedure instead of duplicating it.
   *
   * Is any of this noticeable?  Probably not, so I'll take the
   * smaller code size.
   */
  void
  crit_exit_wrapper(__DEBUG_CRIT_ARG__)
  {
      _crit_exit(mycpu __DEBUG_CRIT_PASS_ARG__);
  }
  
  void
  crit_panic(void)
  {
      thread_t td = curthread;
      int lcrit = td->td_critcount;
  
      td->td_critcount = 0;
      panic("td_critcount is/would-go negative! %p %d", td, lcrit);
      /* NOT REACHED */
  }
  
  /*
   * Called from debugger/panic on cpus which have been stopped.  We must still
   * process the IPIQ while stopped, even if we were stopped while in a critical
   * section (XXX).
   *
   * If we are dumping also try to process any pending interrupts.  This may
   * or may not work depending on the state of the cpu at the point it was
   * stopped.
   */
  void
  lwkt_smp_stopped(void)
  {
      globaldata_t gd = mycpu;
  
      crit_enter_gd(gd);
      if (dumping) {
  	lwkt_process_ipiq();
  	splz();
      } else {
  	lwkt_process_ipiq();
      }
      crit_exit_gd(gd);
  }
  