text: /* Copyright (c) 2017-2019, The Tor Project, Inc. */
  /* See LICENSE for licensing information */
  
  #define SCHEDULER_KIST_PRIVATE
  
  #include "core/or/or.h"
  #include "lib/buf/buffers.h"
  #include "app/config/config.h"
  #include "core/mainloop/connection.h"
  #include "feature/nodelist/networkstatus.h"
  #define TOR_CHANNEL_INTERNAL_
  #include "core/or/channel.h"
  #include "core/or/channeltls.h"
  #define SCHEDULER_PRIVATE_
  #include "core/or/scheduler.h"
  #include "lib/math/fp.h"
  
  #include "core/or/or_connection_st.h"
  
  #ifdef HAVE_SYS_IOCTL_H
  #include <sys/ioctl.h>
  #endif
  
  #ifdef HAVE_KIST_SUPPORT
  /* Kernel interface needed for KIST. */
  #include <netinet/tcp.h>
  #include <linux/sockios.h>
  #endif /* HAVE_KIST_SUPPORT */
  
  /*****************************************************************************
   * Data structures and supporting functions
   *****************************************************************************/
  
  /* Socket_table hash table stuff. The socket_table keeps track of per-socket
   * limit information imposed by kist and used by kist. */
  
  static uint32_t
  socket_table_ent_hash(const socket_table_ent_t *ent)
  {
    return (uint32_t)ent->chan->global_identifier;
  }
  
  static unsigned
  socket_table_ent_eq(const socket_table_ent_t *a, const socket_table_ent_t *b)
  {
    return a->chan == b->chan;
  }
  
  typedef HT_HEAD(socket_table_s, socket_table_ent_s) socket_table_t;
  
  static socket_table_t socket_table = HT_INITIALIZER();
  
  HT_PROTOTYPE(socket_table_s, socket_table_ent_s, node, socket_table_ent_hash,
               socket_table_ent_eq)
  HT_GENERATE2(socket_table_s, socket_table_ent_s, node, socket_table_ent_hash,
               socket_table_ent_eq, 0.6, tor_reallocarray, tor_free_)
  
  /* outbuf_table hash table stuff. The outbuf_table keeps track of which
   * channels have data sitting in their outbuf so the kist scheduler can force
   * a write from outbuf to kernel periodically during a run and at the end of a
   * run. */
  
  typedef struct outbuf_table_ent_s {
    HT_ENTRY(outbuf_table_ent_s) node;
    channel_t *chan;
  } outbuf_table_ent_t;
  
  static uint32_t
  outbuf_table_ent_hash(const outbuf_table_ent_t *ent)
  {
    return (uint32_t)ent->chan->global_identifier;
  }
  
  static unsigned
  outbuf_table_ent_eq(const outbuf_table_ent_t *a, const outbuf_table_ent_t *b)
  {
    return a->chan->global_identifier == b->chan->global_identifier;
  }
  
  HT_PROTOTYPE(outbuf_table_s, outbuf_table_ent_s, node, outbuf_table_ent_hash,
               outbuf_table_ent_eq)
  HT_GENERATE2(outbuf_table_s, outbuf_table_ent_s, node, outbuf_table_ent_hash,
               outbuf_table_ent_eq, 0.6, tor_reallocarray, tor_free_)
  
  /*****************************************************************************
   * Other internal data
   *****************************************************************************/
  
  /* Store the last time the scheduler was run so we can decide when to next run
   * the scheduler based on it. */
  static monotime_t scheduler_last_run;
  /* This is a factor for the extra_space calculation in kist per-socket limits.
   * It is the number of extra congestion windows we want to write to the kernel.
   */
  static double sock_buf_size_factor = 1.0;
  /* How often the scheduler runs. */
  STATIC int sched_run_interval = KIST_SCHED_RUN_INTERVAL_DEFAULT;
  
  #ifdef HAVE_KIST_SUPPORT
  /* Indicate if KIST lite mode is on or off. We can disable it at runtime.
   * Important to have because of the KISTLite -> KIST possible transition. */
  static unsigned int kist_lite_mode = 0;
  /* Indicate if we don't have the kernel support. This can happen if the kernel
   * changed and it doesn't recognized the values passed to the syscalls needed
   * by KIST. In that case, fallback to the naive approach. */
  static unsigned int kist_no_kernel_support = 0;
  #else /* !defined(HAVE_KIST_SUPPORT) */
  static unsigned int kist_lite_mode = 1;
  #endif /* defined(HAVE_KIST_SUPPORT) */
  
  /*****************************************************************************
   * Internally called function implementations
   *****************************************************************************/
  
  /* Little helper function to get the length of a channel's output buffer */
  static inline size_t
  channel_outbuf_length(channel_t *chan)
  {
    tor_assert(chan);
    /* In theory, this can not happen because we can not scheduler a channel
     * without a connection that has its outbuf initialized. Just in case, bug
     * on this so we can understand a bit more why it happened. */
    if (SCHED_BUG(BASE_CHAN_TO_TLS(chan)->conn == NULL, chan)) {
      return 0;
    }
    return buf_datalen(TO_CONN(BASE_CHAN_TO_TLS(chan)->conn)->outbuf);
  }
  
  /* Little helper function for HT_FOREACH_FN. */
  static int
  each_channel_write_to_kernel(outbuf_table_ent_t *ent, void *data)
  {
    (void) data; /* Make compiler happy. */
    channel_write_to_kernel(ent->chan);
    return 0; /* Returning non-zero removes the element from the table. */
  }
  
  /* Free the given outbuf table entry ent. */
  static int
  free_outbuf_info_by_ent(outbuf_table_ent_t *ent, void *data)
  {
    (void) data; /* Make compiler happy. */
    log_debug(LD_SCHED, "Freeing outbuf table entry from chan=%" PRIu64,
              ent->chan->global_identifier);
    tor_free(ent);
    return 1; /* So HT_FOREACH_FN will remove the element */
  }
  
  /* Free the given socket table entry ent. */
  static int
  free_socket_info_by_ent(socket_table_ent_t *ent, void *data)
  {
    (void) data; /* Make compiler happy. */
    log_debug(LD_SCHED, "Freeing socket table entry from chan=%" PRIu64,
              ent->chan->global_identifier);
    tor_free(ent);
    return 1; /* So HT_FOREACH_FN will remove the element */
  }
  
  /* Clean up socket_table. Probably because the KIST sched impl is going away */
  static void
  free_all_socket_info(void)
  {
    HT_FOREACH_FN(socket_table_s, &socket_table, free_socket_info_by_ent, NULL);
    HT_CLEAR(socket_table_s, &socket_table);
  }
  
  static socket_table_ent_t *
  socket_table_search(socket_table_t *table, const channel_t *chan)
  {
    socket_table_ent_t search, *ent = NULL;
    search.chan = chan;
    ent = HT_FIND(socket_table_s, table, &search);
    return ent;
  }
  
  /* Free a socket entry in table for the given chan. */
  static void
  free_socket_info_by_chan(socket_table_t *table, const channel_t *chan)
  {
    socket_table_ent_t *ent = NULL;
    ent = socket_table_search(table, chan);
    if (!ent)
      return;
    log_debug(LD_SCHED, "scheduler free socket info for chan=%" PRIu64,
              chan->global_identifier);
    HT_REMOVE(socket_table_s, table, ent);
    free_socket_info_by_ent(ent, NULL);
  }
  
  /* Perform system calls for the given socket in order to calculate kist's
   * per-socket limit as documented in the function body. */
  MOCK_IMPL(void,
  update_socket_info_impl, (socket_table_ent_t *ent))
  {
  #ifdef HAVE_KIST_SUPPORT
    int64_t tcp_space, extra_space;
    tor_assert(ent);
    tor_assert(ent->chan);
    const tor_socket_t sock =
      TO_CONN(BASE_CHAN_TO_TLS((channel_t *) ent->chan)->conn)->s;
    struct tcp_info tcp;
    socklen_t tcp_info_len = sizeof(tcp);
  
    if (kist_no_kernel_support || kist_lite_mode) {
      goto fallback;
    }
  
    /* Gather information */
    if (getsockopt(sock, SOL_TCP, TCP_INFO, (void *)&(tcp), &tcp_info_len) < 0) {
      if (errno == EINVAL) {
        /* Oops, this option is not provided by the kernel, we'll have to
         * disable KIST entirely. This can happen if tor was built on a machine
         * with the support previously or if the kernel was updated and lost the
         * support. */
        log_notice(LD_SCHED, "Looks like our kernel doesn't have the support "
                             "for KIST anymore. We will fallback to the naive "
                             "approach. Remove KIST from the Schedulers list "
                             "to disable.");
        kist_no_kernel_support = 1;
      }
      goto fallback;
    }
    if (ioctl(sock, SIOCOUTQNSD, &(ent->notsent)) < 0) {
      if (errno == EINVAL) {
        log_notice(LD_SCHED, "Looks like our kernel doesn't have the support "
                             "for KIST anymore. We will fallback to the naive "
                             "approach. Remove KIST from the Schedulers list "
                             "to disable.");
        /* Same reason as the above. */
        kist_no_kernel_support = 1;
      }
      goto fallback;
    }
    ent->cwnd = tcp.tcpi_snd_cwnd;
    ent->unacked = tcp.tcpi_unacked;
    ent->mss = tcp.tcpi_snd_mss;
  
    /* In order to reduce outbound kernel queuing delays and thus improve Tor's
     * ability to prioritize circuits, KIST wants to set a socket write limit
     * that is near the amount that the socket would be able to immediately send
     * into the Internet.
     *
     * We first calculate how much the socket could send immediately (assuming
     * completely full packets) according to the congestion window and the number
     * of unacked packets.
     *
     * Then we add a little extra space in a controlled way. We do this so any
     * when the kernel gets ACKs back for data currently sitting in the "TCP
     * space", it will already have some more data to send immediately. It will
     * not have to wait for the scheduler to run again. The amount of extra space
     * is a factor of the current congestion window. With the suggested
     * sock_buf_size_factor value of 1.0, we allow at most 2*cwnd bytes to sit in
     * the kernel: 1 cwnd on the wire waiting for ACKs and 1 cwnd ready and
     * waiting to be sent when those ACKs finally come.
     *
     * In the below diagram, we see some bytes in the TCP-space (denoted by '*')
     * that have be sent onto the wire and are waiting for ACKs. We have a little
     * more room in "TCP space" that we can fill with data that will be
     * immediately sent. We also see the "extra space" KIST calculates. The sum
     * of the empty "TCP space" and the "extra space" is the kist-imposed write
     * limit for this socket.
     *
     * <----------------kernel-outbound-socket-queue----------------|
     * <*********---------------------------------------------------|
     * |----TCP-space-----|----extra-space-----|
     * |------------------|
     *                    ^ ((cwnd - unacked) * mss) bytes
     *                    |--------------------|
     *                                         ^ ((cwnd * mss) * factor) bytes
     */
  
    /* These values from the kernel are uint32_t, they will always fit into a
     * int64_t tcp_space variable but if the congestion window cwnd is smaller
     * than the unacked packets, the remaining TCP space is set to 0. */
    if (ent->cwnd >= ent->unacked) {
      tcp_space = (ent->cwnd - ent->unacked) * (int64_t)(ent->mss);
    } else {
      tcp_space = 0;
    }
  
    /* The clamp_double_to_int64 makes sure the first part fits into an int64_t.
     * In fact, if sock_buf_size_factor is still forced to be >= 0 in config.c,
     * then it will be positive for sure. Then we subtract a uint32_t. Getting a
     * negative value is OK, see after how it is being handled. */
    extra_space =
      clamp_double_to_int64(
                   (ent->cwnd * (int64_t)ent->mss) * sock_buf_size_factor) -
      ent->notsent - (int64_t)channel_outbuf_length((channel_t *) ent->chan);
    if ((tcp_space + extra_space) < 0) {
      /* This means that the "notsent" queue is just too big so we shouldn't put
       * more in the kernel for now. */
      ent->limit = 0;
    } else {
      /* The positive sum of two int64_t will always fit into an uint64_t.
       * And we know this will always be positive, since we checked above. */
      ent->limit = (uint64_t)tcp_space + (uint64_t)extra_space;
    }
    return;
  
  #else /* !defined(HAVE_KIST_SUPPORT) */
    goto fallback;
  #endif /* defined(HAVE_KIST_SUPPORT) */
  
   fallback:
    /* If all of a sudden we don't have kist support, we just zero out all the
     * variables for this socket since we don't know what they should be. We
     * also allow the socket to write as much as it can from the estimated
     * number of cells the lower layer can accept, effectively returning it to
     * Vanilla scheduler behavior. */
    ent->cwnd = ent->unacked = ent->mss = ent->notsent = 0;
    /* This function calls the specialized channel object (currently channeltls)
     * and ask how many cells it can write on the outbuf which we then multiply
     * by the size of the cells for this channel. The cast is because this
     * function requires a non-const channel object, meh. */
    ent->limit = channel_num_cells_writeable((channel_t *) ent->chan) *
                 (get_cell_network_size(ent->chan->wide_circ_ids) +
                  TLS_PER_CELL_OVERHEAD);
  }
  
  /* Given a socket that isn't in the table, add it.
   * Given a socket that is in the table, re-init values that need init-ing
   * every scheduling run
   */
  static void
  init_socket_info(socket_table_t *table, const channel_t *chan)
  {
    socket_table_ent_t *ent = NULL;
    ent = socket_table_search(table, chan);
    if (!ent) {
      log_debug(LD_SCHED, "scheduler init socket info for chan=%" PRIu64,
                chan->global_identifier);
      ent = tor_malloc_zero(sizeof(*ent));
      ent->chan = chan;
      HT_INSERT(socket_table_s, table, ent);
    }
    ent->written = 0;
  }
  
  /* Add chan to the outbuf table if it isn't already in it. If it is, then don't
   * do anything */
  static void
  outbuf_table_add(outbuf_table_t *table, channel_t *chan)
  {
    outbuf_table_ent_t search, *ent;
    search.chan = chan;
    ent = HT_FIND(outbuf_table_s, table, &search);
    if (!ent) {
      log_debug(LD_SCHED, "scheduler init outbuf info for chan=%" PRIu64,
                chan->global_identifier);
      ent = tor_malloc_zero(sizeof(*ent));
      ent->chan = chan;
      HT_INSERT(outbuf_table_s, table, ent);
    }
  }
  
  static void
  outbuf_table_remove(outbuf_table_t *table, channel_t *chan)
  {
    outbuf_table_ent_t search, *ent;
    search.chan = chan;
    ent = HT_FIND(outbuf_table_s, table, &search);
    if (ent) {
      HT_REMOVE(outbuf_table_s, table, ent);
      free_outbuf_info_by_ent(ent, NULL);
    }
  }
  
  /* Set the scheduler running interval. */
  static void
  set_scheduler_run_interval(void)
  {
    int old_sched_run_interval = sched_run_interval;
    sched_run_interval = kist_scheduler_run_interval();
    if (old_sched_run_interval != sched_run_interval) {
      log_info(LD_SCHED, "Scheduler KIST changing its running interval "
                         "from %" PRId32 " to %" PRId32,
               old_sched_run_interval, sched_run_interval);
    }
  }
  
  /* Return true iff the channel hasn't hit its kist-imposed write limit yet */
  static int
  socket_can_write(socket_table_t *table, const channel_t *chan)
  {
    socket_table_ent_t *ent = NULL;
    ent = socket_table_search(table, chan);
    if (SCHED_BUG(!ent, chan)) {
      return 1; // Just return true, saying that kist wouldn't limit the socket
    }
  
    /* We previously calculated a write limit for this socket. In the below
     * calculation, first determine how much room is left in bytes. Then divide
     * that by the amount of space a cell takes. If there's room for at least 1
     * cell, then KIST will allow the socket to write. */
    int64_t kist_limit_space =
      (int64_t) (ent->limit - ent->written) /
      (CELL_MAX_NETWORK_SIZE + TLS_PER_CELL_OVERHEAD);
    return kist_limit_space > 0;
  }
  
  /* Update the channel's socket kernel information. */
  static void
  update_socket_info(socket_table_t *table, const channel_t *chan)
  {
    socket_table_ent_t *ent = NULL;
    ent = socket_table_search(table, chan);
    if (SCHED_BUG(!ent, chan)) {
      return; // Whelp. Entry didn't exist for some reason so nothing to do.
    }
    update_socket_info_impl(ent);
    log_debug(LD_SCHED, "chan=%" PRIu64 " updated socket info, limit: %" PRIu64
                        ", cwnd: %" PRIu32 ", unacked: %" PRIu32
                        ", notsent: %" PRIu32 ", mss: %" PRIu32,
              ent->chan->global_identifier, ent->limit, ent->cwnd, ent->unacked,
              ent->notsent, ent->mss);
  }
  
  /* Increment the channel's socket written value by the number of bytes. */
  static void
  update_socket_written(socket_table_t *table, channel_t *chan, size_t bytes)
  {
    socket_table_ent_t *ent = NULL;
    ent = socket_table_search(table, chan);
    if (SCHED_BUG(!ent, chan)) {
      return; // Whelp. Entry didn't exist so nothing to do.
    }
  
    log_debug(LD_SCHED, "chan=%" PRIu64 " wrote %lu bytes, old was %" PRIi64,
              chan->global_identifier, (unsigned long) bytes, ent->written);
  
    ent->written += bytes;
  }
  
  /*
   * A naive KIST impl would write every single cell all the way to the kernel.
   * That would take a lot of system calls. A less bad KIST impl would write a
   * channel's outbuf to the kernel only when we are switching to a different
   * channel. But if we have two channels with equal priority, we end up writing
   * one cell for each and bouncing back and forth. This KIST impl avoids that
   * by only writing a channel's outbuf to the kernel if it has 8 cells or more
   * in it.
   */
  MOCK_IMPL(int, channel_should_write_to_kernel,
            (outbuf_table_t *table, channel_t *chan))
  {
    outbuf_table_add(table, chan);
    /* CELL_MAX_NETWORK_SIZE * 8 because we only want to write the outbuf to the
     * kernel if there's 8 or more cells waiting */
    return channel_outbuf_length(chan) > (CELL_MAX_NETWORK_SIZE * 8);
  }
  
  /* Little helper function to write a channel's outbuf all the way to the
   * kernel */
  MOCK_IMPL(void, channel_write_to_kernel, (channel_t *chan))
  {
    tor_assert(chan);
    log_debug(LD_SCHED, "Writing %lu bytes to kernel for chan %" PRIu64,
              (unsigned long)channel_outbuf_length(chan),
              chan->global_identifier);
    connection_handle_write(TO_CONN(BASE_CHAN_TO_TLS(chan)->conn), 0);
  }
  
  /* Return true iff the scheduler has work to perform. */
  static int
  have_work(void)
  {
    smartlist_t *cp = get_channels_pending();
    IF_BUG_ONCE(!cp) {
      return 0; // channels_pending doesn't exist so... no work?
    }
    return smartlist_len(cp) > 0;
  }
  
  /* Function of the scheduler interface: free_all() */
  static void
  kist_free_all(void)
  {
    free_all_socket_info();
  }
  
  /* Function of the scheduler interface: on_channel_free() */
  static void
  kist_on_channel_free_fn(const channel_t *chan)
  {
    free_socket_info_by_chan(&socket_table, chan);
  }
  
  /* Function of the scheduler interface: on_new_consensus() */
  static void
  kist_scheduler_on_new_consensus(void)
  {
    set_scheduler_run_interval();
  }
  
  /* Function of the scheduler interface: on_new_options() */
  static void
  kist_scheduler_on_new_options(void)
  {
    sock_buf_size_factor = get_options()->KISTSockBufSizeFactor;
  
    /* Calls kist_scheduler_run_interval which calls get_options(). */
    set_scheduler_run_interval();
  }
  
  /* Function of the scheduler interface: init() */
  static void
  kist_scheduler_init(void)
  {
    /* When initializing the scheduler, the last run could be 0 because it is
     * declared static or a value in the past that was set when it was last
     * used. In both cases, we want to initialize it to now so we don't risk
     * using the value 0 which doesn't play well with our monotonic time
     * interface.
     *
     * One side effect is that the first scheduler run will be at the next tick
     * that is in now + 10 msec (KIST_SCHED_RUN_INTERVAL_DEFAULT) by default. */
    monotime_get(&scheduler_last_run);
  
    kist_scheduler_on_new_options();
    IF_BUG_ONCE(sched_run_interval == 0) {
      log_warn(LD_SCHED, "We are initing the KIST scheduler and noticed the "
               "KISTSchedRunInterval is telling us to not use KIST. That's "
               "weird! We'll continue using KIST, but at %" PRId32 "ms.",
               KIST_SCHED_RUN_INTERVAL_DEFAULT);
      sched_run_interval = KIST_SCHED_RUN_INTERVAL_DEFAULT;
    }
  }
  
  /* Function of the scheduler interface: schedule() */
  static void
  kist_scheduler_schedule(void)
  {
    struct monotime_t now;
    struct timeval next_run;
    int64_t diff;
  
    if (!have_work()) {
      return;
    }
    monotime_get(&now);
  
    /* If time is really monotonic, we can never have now being smaller than the
     * last scheduler run. The scheduler_last_run at first is set to 0.
     * Unfortunately, not all platforms guarantee monotonic time so we log at
     * info level but don't make it more noisy. */
    diff = monotime_diff_msec(&scheduler_last_run, &now);
    if (diff < 0) {
      log_info(LD_SCHED, "Monotonic time between now and last run of scheduler "
                         "is negative: %" PRId64 ". Setting diff to 0.", diff);
      diff = 0;
    }
    if (diff < sched_run_interval) {
      next_run.tv_sec = 0;
      /* Takes 1000 ms -> us. This will always be valid because diff can NOT be
       * negative and can NOT be bigger than sched_run_interval so values can
       * only go from 1000 usec (diff set to interval - 1) to 100000 usec (diff
       * set to 0) for the maximum allowed run interval (100ms). */
      next_run.tv_usec = (int) ((sched_run_interval - diff) * 1000);
      /* Re-adding an event reschedules it. It does not duplicate it. */
      scheduler_ev_add(&next_run);
    } else {
      scheduler_ev_active();
    }
  }
  
  /* Function of the scheduler interface: run() */
  static void
  kist_scheduler_run(void)
  {
    /* Define variables */
    channel_t *chan = NULL; // current working channel
    /* The last distinct chan served in a sched loop. */
    channel_t *prev_chan = NULL;
    int flush_result; // temporarily store results from flush calls
    /* Channels to be re-adding to pending at the end */
    smartlist_t *to_readd = NULL;
    smartlist_t *cp = get_channels_pending();
  
    outbuf_table_t outbuf_table = HT_INITIALIZER();
  
    /* For each pending channel, collect new kernel information */
    SMARTLIST_FOREACH_BEGIN(cp, const channel_t *, pchan) {
        init_socket_info(&socket_table, pchan);
        update_socket_info(&socket_table, pchan);
    } SMARTLIST_FOREACH_END(pchan);
  
    log_debug(LD_SCHED, "Running the scheduler. %d channels pending",
              smartlist_len(cp));
  
    /* The main scheduling loop. Loop until there are no more pending channels */
    while (smartlist_len(cp) > 0) {
      /* get best channel */
      chan = smartlist_pqueue_pop(cp, scheduler_compare_channels,
                                  offsetof(channel_t, sched_heap_idx));
      if (SCHED_BUG(!chan, NULL)) {
        /* Some-freaking-how a NULL got into the channels_pending. That should
         * never happen, but it should be harmless to ignore it and keep looping.
         */
        continue;
      }
      outbuf_table_add(&outbuf_table, chan);
  
      /* if we have switched to a new channel, consider writing the previous
       * channel's outbuf to the kernel. */
      if (!prev_chan) {
        prev_chan = chan;
      }
      if (prev_chan != chan) {
        if (channel_should_write_to_kernel(&outbuf_table, prev_chan)) {
          channel_write_to_kernel(prev_chan);
          outbuf_table_remove(&outbuf_table, prev_chan);
        }
        prev_chan = chan;
      }
  
      /* Only flush and write if the per-socket limit hasn't been hit */
      if (socket_can_write(&socket_table, chan)) {
        /* flush to channel queue/outbuf */
        flush_result = (int)channel_flush_some_cells(chan, 1); // 1 for num cells
        /* XXX: While flushing cells, it is possible that the connection write
         * fails leading to the channel to be closed which triggers a release
         * and free its entry in the socket table. And because of a engineering
         * design issue, the error is not propagated back so we don't get an
         * error at this point. So before we continue, make sure the channel is
         * open and if not just ignore it. See #23751. */
        if (!CHANNEL_IS_OPEN(chan)) {
          /* Channel isn't open so we put it back in IDLE mode. It is either
           * renegotiating its TLS session or about to be released. */
          scheduler_set_channel_state(chan, SCHED_CHAN_IDLE);
          continue;
        }
        /* flush_result has the # cells flushed */
        if (flush_result > 0) {
          update_socket_written(&socket_table, chan, flush_result *
                                (CELL_MAX_NETWORK_SIZE + TLS_PER_CELL_OVERHEAD));
        } else {
          /* XXX: This can happen because tor sometimes does flush in an
           * opportunistic way cells from the circuit to the outbuf so the
           * channel can end up here without having anything to flush nor needed
           * to write to the kernel. Hopefully we'll fix that soon but for now
           * we have to handle this case which happens kind of often. */
          log_debug(LD_SCHED,
                   "We didn't flush anything on a chan that we think "
                   "can write and wants to write. The channel's state is '%s' "
                   "and in scheduler state '%s'. We're going to mark it as "
                   "waiting_for_cells (as that's most likely the issue) and "
                   "stop scheduling it this round.",
                   channel_state_to_string(chan->state),
                   get_scheduler_state_string(chan->scheduler_state));
          scheduler_set_channel_state(chan, SCHED_CHAN_WAITING_FOR_CELLS);
          continue;
        }
      }
  
      /* Decide what to do with the channel now */
  
      if (!channel_more_to_flush(chan) &&
          !socket_can_write(&socket_table, chan)) {
  
        /* Case 1: no more cells to send, and cannot write */
  
        /*
         * You might think we should put the channel in SCHED_CHAN_IDLE. And
         * you're probably correct. While implementing KIST, we found that the
         * scheduling system would sometimes lose track of channels when we did
         * that. We suspect it has to do with the difference between "can't
         * write because socket/outbuf is full" and KIST's "can't write because
         * we've arbitrarily decided that that's enough for now." Sometimes
         * channels run out of cells at the same time they hit their
         * kist-imposed write limit and maybe the rest of Tor doesn't put the
         * channel back in pending when it is supposed to.
         *
         * This should be investigated again. It is as simple as changing
         * SCHED_CHAN_WAITING_FOR_CELLS to SCHED_CHAN_IDLE and seeing if Tor
         * starts having serious throughput issues. Best done in shadow/chutney.
         */
        scheduler_set_channel_state(chan, SCHED_CHAN_WAITING_FOR_CELLS);
      } else if (!channel_more_to_flush(chan)) {
  
        /* Case 2: no more cells to send, but still open for writes */
  
        scheduler_set_channel_state(chan, SCHED_CHAN_WAITING_FOR_CELLS);
      } else if (!socket_can_write(&socket_table, chan)) {
  
        /* Case 3: cells to send, but cannot write */
  
        /*
         * We want to write, but can't. If we left the channel in
         * channels_pending, we would never exit the scheduling loop. We need to
         * add it to a temporary list of channels to be added to channels_pending
         * after the scheduling loop is over. They can hopefully be taken care of
         * in the next scheduling round.
         */
        if (!to_readd) {
          to_readd = smartlist_new();
        }
        smartlist_add(to_readd, chan);
      } else {
  
        /* Case 4: cells to send, and still open for writes */
  
        scheduler_set_channel_state(chan, SCHED_CHAN_PENDING);
        if (!SCHED_BUG(chan->sched_heap_idx != -1, chan)) {
          smartlist_pqueue_add(cp, scheduler_compare_channels,
                               offsetof(channel_t, sched_heap_idx), chan);
        }
      }
    } /* End of main scheduling loop */
  
    /* Write the outbuf of any channels that still have data */
    HT_FOREACH_FN(outbuf_table_s, &outbuf_table, each_channel_write_to_kernel,
                  NULL);
    /* We are done with it. */
    HT_FOREACH_FN(outbuf_table_s, &outbuf_table, free_outbuf_info_by_ent, NULL);
    HT_CLEAR(outbuf_table_s, &outbuf_table);
  
    log_debug(LD_SCHED, "len pending=%d, len to_readd=%d",
              smartlist_len(cp),
              (to_readd ? smartlist_len(to_readd) : -1));
  
    /* Re-add any channels we need to */
    if (to_readd) {
      SMARTLIST_FOREACH_BEGIN(to_readd, channel_t *, readd_chan) {
        scheduler_set_channel_state(readd_chan, SCHED_CHAN_PENDING);
        if (!smartlist_contains(cp, readd_chan)) {
          if (!SCHED_BUG(readd_chan->sched_heap_idx != -1, readd_chan)) {
            /* XXXX Note that the check above is in theory redundant with
             * the smartlist_contains check.  But let's make sure we're
             * not messing anything up, and leave them both for now. */
            smartlist_pqueue_add(cp, scheduler_compare_channels,
                               offsetof(channel_t, sched_heap_idx), readd_chan);
          }
        }
      } SMARTLIST_FOREACH_END(readd_chan);
      smartlist_free(to_readd);
    }
  
    monotime_get(&scheduler_last_run);
  }
  
  /*****************************************************************************
   * Externally called function implementations not called through scheduler_t
   *****************************************************************************/
  
  /* Stores the kist scheduler function pointers. */
  static scheduler_t kist_scheduler = {
    .type = SCHEDULER_KIST,
    .free_all = kist_free_all,
    .on_channel_free = kist_on_channel_free_fn,
    .init = kist_scheduler_init,
    .on_new_consensus = kist_scheduler_on_new_consensus,
    .schedule = kist_scheduler_schedule,
    .run = kist_scheduler_run,
    .on_new_options = kist_scheduler_on_new_options,
  };
  
  /* Return the KIST scheduler object. If it didn't exists, return a newly
   * allocated one but init() is not called. */
  scheduler_t *
  get_kist_scheduler(void)
  {
    return &kist_scheduler;
  }
  
  /* Check the torrc (and maybe consensus) for the configured KIST scheduler run
   * interval.
   * - If torrc > 0, then return the positive torrc value (should use KIST, and
   *   should use the set value)
   * - If torrc == 0, then look in the consensus for what the value should be.
   *   - If == 0, then return 0 (don't use KIST)
   *   - If > 0, then return the positive consensus value
   *   - If consensus doesn't say anything, return 10 milliseconds, default.
   */
  int
  kist_scheduler_run_interval(void)
  {
    int run_interval = get_options()->KISTSchedRunInterval;
  
    if (run_interval != 0) {
      log_debug(LD_SCHED, "Found KISTSchedRunInterval=%" PRId32 " in torrc. "
                          "Using that.", run_interval);
      return run_interval;
    }
  
    log_debug(LD_SCHED, "KISTSchedRunInterval=0, turning to the consensus.");
  
    /* Will either be the consensus value or the default. Note that 0 can be
     * returned which means the consensus wants us to NOT use KIST. */
    return networkstatus_get_param(NULL, "KISTSchedRunInterval",
                                   KIST_SCHED_RUN_INTERVAL_DEFAULT,
                                   KIST_SCHED_RUN_INTERVAL_MIN,
                                   KIST_SCHED_RUN_INTERVAL_MAX);
  }
  
  /* Set KISTLite mode that is KIST without kernel support. */
  void
  scheduler_kist_set_lite_mode(void)
  {
    kist_lite_mode = 1;
    kist_scheduler.type = SCHEDULER_KIST_LITE;
    log_info(LD_SCHED,
             "Setting KIST scheduler without kernel support (KISTLite mode)");
  }
  
  /* Set KIST mode that is KIST with kernel support. */
  void
  scheduler_kist_set_full_mode(void)
  {
    kist_lite_mode = 0;
    kist_scheduler.type = SCHEDULER_KIST;
    log_info(LD_SCHED,
             "Setting KIST scheduler with kernel support (KIST mode)");
  }
  
  #ifdef HAVE_KIST_SUPPORT
  
  /* Return true iff the scheduler subsystem should use KIST. */
  int
  scheduler_can_use_kist(void)
  {
    if (kist_no_kernel_support) {
      /* We have no kernel support so we can't use KIST. */
      return 0;
    }
  
    /* We do have the support, time to check if we can get the interval that the
     * consensus can be disabling. */
    int run_interval = kist_scheduler_run_interval();
    log_debug(LD_SCHED, "Determined KIST sched_run_interval should be "
                        "%" PRId32 ". Can%s use KIST.",
             run_interval, (run_interval > 0 ? "" : " not"));
    return run_interval > 0;
  }
  
  #else /* !defined(HAVE_KIST_SUPPORT) */
  
  int
  scheduler_can_use_kist(void)
  {
    return 0;
  }
  
  #endif /* defined(HAVE_KIST_SUPPORT) */
  