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[tor-commits] [tor/maint-0.3.5] Merge branch 'maint-0.3.4' into maint-0.3.5



commit 4a8a1f76ea6cbde128f2cda49049866ea739931d
Merge: c663716b6 506264791
Author: Nick Mathewson <nickm@xxxxxxxxxxxxxx>
Date:   Thu Feb 21 10:08:14 2019 -0500

    Merge branch 'maint-0.3.4' into maint-0.3.5

 changes/ticket29168          | 5 +++++
 src/core/or/scheduler_kist.c | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --cc src/core/or/scheduler_kist.c
index e17d67cfd,000000000..79ecb0bc7
mode 100644,000000..100644
--- a/src/core/or/scheduler_kist.c
+++ b/src/core/or/scheduler_kist.c
@@@ -1,844 -1,0 +1,844 @@@
 +/* Copyright (c) 2017-2019, The Tor Project, Inc. */
 +/* See LICENSE for licensing information */
 +
 +#define SCHEDULER_KIST_PRIVATE
 +
 +#include "core/or/or.h"
 +#include "lib/container/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;
++    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(chan->sched_heap_idx != -1, 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) */

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