text: // SPDX-License-Identifier: GPL-2.0-only
  /*
   *  kernel/sched/cpupri.c
   *
   *  CPU priority management
   *
   *  Copyright (C) 2007-2008 Novell
   *
   *  Author: Gregory Haskins <ghaskins@novell.com>
   *
   *  This code tracks the priority of each CPU so that global migration
   *  decisions are easy to calculate.  Each CPU can be in a state as follows:
   *
   *                 (INVALID), IDLE, NORMAL, RT1, ... RT99
   *
   *  going from the lowest priority to the highest.  CPUs in the INVALID state
   *  are not eligible for routing.  The system maintains this state with
   *  a 2 dimensional bitmap (the first for priority class, the second for CPUs
   *  in that class).  Therefore a typical application without affinity
   *  restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
   *  searches).  For tasks with affinity restrictions, the algorithm has a
   *  worst case complexity of O(min(102, nr_domcpus)), though the scenario that
   *  yields the worst case search is fairly contrived.
   */
  #include "sched.h"
  
  /* Convert between a 140 based task->prio, and our 102 based cpupri */
  static int convert_prio(int prio)
  {
  	int cpupri;
  
  	if (prio == CPUPRI_INVALID)
  		cpupri = CPUPRI_INVALID;
  	else if (prio == MAX_PRIO)
  		cpupri = CPUPRI_IDLE;
  	else if (prio >= MAX_RT_PRIO)
  		cpupri = CPUPRI_NORMAL;
  	else
  		cpupri = MAX_RT_PRIO - prio + 1;
  
  	return cpupri;
  }
  
  /**
   * cpupri_find - find the best (lowest-pri) CPU in the system
   * @cp: The cpupri context
   * @p: The task
   * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
   *
   * Note: This function returns the recommended CPUs as calculated during the
   * current invocation.  By the time the call returns, the CPUs may have in
   * fact changed priorities any number of times.  While not ideal, it is not
   * an issue of correctness since the normal rebalancer logic will correct
   * any discrepancies created by racing against the uncertainty of the current
   * priority configuration.
   *
   * Return: (int)bool - CPUs were found
   */
  int cpupri_find(struct cpupri *cp, struct task_struct *p,
  		struct cpumask *lowest_mask)
  {
  	int idx = 0;
  	int task_pri = convert_prio(p->prio);
  
  	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
  
  	for (idx = 0; idx < task_pri; idx++) {
  		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
  		int skip = 0;
  
  		if (!atomic_read(&(vec)->count))
  			skip = 1;
  		/*
  		 * When looking at the vector, we need to read the counter,
  		 * do a memory barrier, then read the mask.
  		 *
  		 * Note: This is still all racey, but we can deal with it.
  		 *  Ideally, we only want to look at masks that are set.
  		 *
  		 *  If a mask is not set, then the only thing wrong is that we
  		 *  did a little more work than necessary.
  		 *
  		 *  If we read a zero count but the mask is set, because of the
  		 *  memory barriers, that can only happen when the highest prio
  		 *  task for a run queue has left the run queue, in which case,
  		 *  it will be followed by a pull. If the task we are processing
  		 *  fails to find a proper place to go, that pull request will
  		 *  pull this task if the run queue is running at a lower
  		 *  priority.
  		 */
  		smp_rmb();
  
  		/* Need to do the rmb for every iteration */
  		if (skip)
  			continue;
  
  		if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
  			continue;
  
  		if (lowest_mask) {
  			cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
  
  			/*
  			 * We have to ensure that we have at least one bit
  			 * still set in the array, since the map could have
  			 * been concurrently emptied between the first and
  			 * second reads of vec->mask.  If we hit this
  			 * condition, simply act as though we never hit this
  			 * priority level and continue on.
  			 */
  			if (cpumask_any(lowest_mask) >= nr_cpu_ids)
  				continue;
  		}
  
  		return 1;
  	}
  
  	return 0;
  }
  
  /**
   * cpupri_set - update the CPU priority setting
   * @cp: The cpupri context
   * @cpu: The target CPU
   * @newpri: The priority (INVALID-RT99) to assign to this CPU
   *
   * Note: Assumes cpu_rq(cpu)->lock is locked
   *
   * Returns: (void)
   */
  void cpupri_set(struct cpupri *cp, int cpu, int newpri)
  {
  	int *currpri = &cp->cpu_to_pri[cpu];
  	int oldpri = *currpri;
  	int do_mb = 0;
  
  	newpri = convert_prio(newpri);
  
  	BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
  
  	if (newpri == oldpri)
  		return;
  
  	/*
  	 * If the CPU was currently mapped to a different value, we
  	 * need to map it to the new value then remove the old value.
  	 * Note, we must add the new value first, otherwise we risk the
  	 * cpu being missed by the priority loop in cpupri_find.
  	 */
  	if (likely(newpri != CPUPRI_INVALID)) {
  		struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
  
  		cpumask_set_cpu(cpu, vec->mask);
  		/*
  		 * When adding a new vector, we update the mask first,
  		 * do a write memory barrier, and then update the count, to
  		 * make sure the vector is visible when count is set.
  		 */
  		smp_mb__before_atomic();
  		atomic_inc(&(vec)->count);
  		do_mb = 1;
  	}
  	if (likely(oldpri != CPUPRI_INVALID)) {
  		struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
  
  		/*
  		 * Because the order of modification of the vec->count
  		 * is important, we must make sure that the update
  		 * of the new prio is seen before we decrement the
  		 * old prio. This makes sure that the loop sees
  		 * one or the other when we raise the priority of
  		 * the run queue. We don't care about when we lower the
  		 * priority, as that will trigger an rt pull anyway.
  		 *
  		 * We only need to do a memory barrier if we updated
  		 * the new priority vec.
  		 */
  		if (do_mb)
  			smp_mb__after_atomic();
  
  		/*
  		 * When removing from the vector, we decrement the counter first
  		 * do a memory barrier and then clear the mask.
  		 */
  		atomic_dec(&(vec)->count);
  		smp_mb__after_atomic();
  		cpumask_clear_cpu(cpu, vec->mask);
  	}
  
  	*currpri = newpri;
  }
  
  /**
   * cpupri_init - initialize the cpupri structure
   * @cp: The cpupri context
   *
   * Return: -ENOMEM on memory allocation failure.
   */
  int cpupri_init(struct cpupri *cp)
  {
  	int i;
  
  	for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
  		struct cpupri_vec *vec = &cp->pri_to_cpu[i];
  
  		atomic_set(&vec->count, 0);
  		if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
  			goto cleanup;
  	}
  
  	cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
  	if (!cp->cpu_to_pri)
  		goto cleanup;
  
  	for_each_possible_cpu(i)
  		cp->cpu_to_pri[i] = CPUPRI_INVALID;
  
  	return 0;
  
  cleanup:
  	for (i--; i >= 0; i--)
  		free_cpumask_var(cp->pri_to_cpu[i].mask);
  	return -ENOMEM;
  }
  
  /**
   * cpupri_cleanup - clean up the cpupri structure
   * @cp: The cpupri context
   */
  void cpupri_cleanup(struct cpupri *cp)
  {
  	int i;
  
  	kfree(cp->cpu_to_pri);
  	for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
  		free_cpumask_var(cp->pri_to_cpu[i].mask);
  }
  