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- /* * Copyright (c) 2013, The Tor Project, Inc. */
- /* See LICENSE for licensing information */
- /**
- * \file scheduler.c
- * \brief Relay scheduling system
- **/
- #include "or.h"
- #include "channel.h"
- #include "compat_libevent.h"
- #include "scheduler.h"
- #ifdef HAVE_EVENT2_EVENT_H
- #include <event2/event.h>
- #else
- #include <event.h>
- #endif
- /*
- * Write scheduling works by keeping track of lists of channels that can
- * accept cells, and have cells to write. From the scheduler's perspective,
- * a channel can be in four possible states:
- *
- * 1.) Not open for writes, no cells to send
- * - Not much to do here, and the channel will appear in neither list.
- * - Transitions from:
- * - Open for writes/has cells by simultaneously draining all circuit
- * queues and filling the output buffer.
- * - Transitions to:
- * - Not open for writes/has cells by arrival of cells on an attached
- * circuit (this would be driven from append_cell_to_circuit_queue())
- * - Open for writes/no cells by a channel type specific path;
- * driven from connection_or_flushed_some() for channel_tls_t.
- *
- * 2.) Open for writes, no cells to send
- * - Not much here either; this will be the state an idle but open channel
- * can be expected to settle in.
- * - Transitions from:
- * - Not open for writes/no cells by flushing some of the output
- * buffer.
- * - Open for writes/has cells by the scheduler moving cells from
- * circuit queues to channel output queue, but not having enough
- * to fill the output queue.
- * - Transitions to:
- * - Open for writes/has cells by arrival of new cells on an attached
- * circuit, in append_cell_to_circuit_queue()
- *
- * 3.) Not open for writes, cells to send
- * - This is the state of a busy circuit limited by output bandwidth;
- * cells have piled up in the circuit queues waiting to be relayed.
- * - Transitions from:
- * - Not open for writes/no cells by arrival of cells on an attached
- * circuit
- * - Open for writes/has cells by filling an output buffer without
- * draining all cells from attached circuits
- * - Transitions to:
- * - Opens for writes/has cells by draining some of the output buffer
- * via the connection_or_flushed_some() path (for channel_tls_t).
- *
- * 4.) Open for writes, cells to send
- * - This connection is ready to relay some cells and waiting for
- * the scheduler to choose it
- * - Transitions from:
- * - Not open for writes/has cells by the connection_or_flushed_some()
- * path
- * - Open for writes/no cells by the append_cell_to_circuit_queue()
- * path
- * - Transitions to:
- * - Not open for writes/no cells by draining all circuit queues and
- * simultaneously filling the output buffer.
- * - Not open for writes/has cells by writing enough cells to fill the
- * output buffer
- * - Open for writes/no cells by draining all attached circuit queues
- * without also filling the output buffer
- *
- * Other event-driven parts of the code move channels between these scheduling
- * states by calling scheduler functions; the scheduler only runs on open-for-
- * writes/has-cells channels and is the only path for those to transition to
- * other states. The scheduler_run() function gives us the opportunity to do
- * scheduling work, and is called from other scheduler functions whenever a
- * state transition occurs, and periodically from the main event loop.
- */
- /* Scheduler global data structures */
- /*
- * We keep lists of channels that either have cells queued, can accept
- * writes, or both (states 2, 3 and 4 above) - no explicit list of state
- * 1 channels is kept, so we don't have to worry about registering new
- * channels here or anything. The scheduler will learn about them when
- * it needs to. We can check how many channels in state 4 in O(1), so
- * the test whether we have anything to do in scheduler_run() is fast
- * and there's no harm in calling it opportunistically whenever we get
- * the chance.
- *
- * Note that it takes time O(n) to search for a channel in these smartlists
- * or move one; I don't think the number of channels on a relay will be large
- * enough for this to be a severe problem, but this would benefit from using
- * a doubly-linked list rather than smartlist_t, together with a hash map from
- * channel identifiers to pointers to list entries, so we can perform those
- * operations in O(log(n)).
- */
- /* List of channels that can write but have no cells (state 2 above) */
- static smartlist_t *channels_waiting_for_cells = NULL;
- /* List of channels with cells waiting to write (state 3 above) */
- static smartlist_t *channels_waiting_to_write = NULL;
- /* List of channels that can write and have cells (pending work) */
- static smartlist_t *channels_pending = NULL;
- /*
- * This event runs the scheduler from its callback, and is manually
- * activated whenever a channel enters open for writes/cells to send.
- */
- static struct event *run_sched_ev = NULL;
- static struct timeval run_sched_tv;
- /* Scheduler static function declarations */
- static void scheduler_evt_callback(evutil_socket_t fd,
- short events, void *arg);
- static int scheduler_more_work(void);
- static void scheduler_retrigger(void);
- static void scheduler_trigger(void);
- /* Scheduler function implementations */
- /** Free everything and shut down the scheduling system */
- void
- scheduler_free_all(void)
- {
- log_debug(LD_SCHED, "Shutting down scheduler");
- if (run_sched_ev) {
- event_del(run_sched_ev);
- tor_event_free(run_sched_ev);
- run_sched_ev = NULL;
- }
- if (channels_waiting_for_cells) {
- smartlist_free(channels_waiting_for_cells);
- channels_waiting_for_cells = NULL;
- }
- if (channels_waiting_to_write) {
- smartlist_free(channels_waiting_to_write);
- channels_waiting_to_write = NULL;
- }
- if (channels_pending) {
- smartlist_free(channels_pending);
- channels_pending = NULL;
- }
- }
- /*
- * Scheduler event callback; this should get triggered once per event loop
- * if any scheduling work was created during the event loop.
- */
- static void
- scheduler_evt_callback(evutil_socket_t fd, short events, void *arg)
- {
- log_debug(LD_SCHED, "Scheduler event callback called");
- tor_assert(run_sched_ev);
- /* Run the scheduler */
- scheduler_run();
- /* Do we have more work to do? */
- if (scheduler_more_work()) scheduler_retrigger();
- }
- /** Mark a channel as no longer ready to accept writes */
- void
- scheduler_channel_doesnt_want_writes(channel_t *chan)
- {
- tor_assert(chan);
- tor_assert(channels_waiting_for_cells);
- tor_assert(channels_waiting_to_write);
- tor_assert(channels_pending);
- /* If it's already in pending, we can put it in waiting_to_write */
- if (smartlist_contains(channels_pending, chan)) {
- /*
- * It's in channels_pending, so it shouldn't be in any of
- * the other lists. It can't write any more, so it goes to
- * channels_waiting_to_write.
- */
- smartlist_remove(channels_pending, chan);
- smartlist_add(channels_waiting_to_write, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p went from pending "
- "to waiting_to_write",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- } else {
- /*
- * It's not in pending, so it can't become waiting_to_write; it's
- * either not in any of the lists (nothing to do) or it's already in
- * waiting_for_cells (remove it, can't write any more).
- */
- if (smartlist_contains(channels_waiting_for_cells, chan)) {
- smartlist_remove(channels_waiting_for_cells, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p left waiting_for_cells",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- }
- }
- }
- /** Mark a channel as having waiting cells */
- void
- scheduler_channel_has_waiting_cells(channel_t *chan)
- {
- int became_pending = 0;
- tor_assert(chan);
- tor_assert(channels_waiting_for_cells);
- tor_assert(channels_waiting_to_write);
- tor_assert(channels_pending);
- /* First, check if this one also writeable */
- if (smartlist_contains(channels_waiting_for_cells, chan)) {
- /*
- * It's in channels_waiting_for_cells, so it shouldn't be in any of
- * the other lists. It has waiting cells now, so it goes to
- * channels_pending.
- */
- smartlist_remove(channels_waiting_for_cells, chan);
- smartlist_add(channels_pending, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p went from waiting_for_cells "
- "to pending",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- became_pending = 1;
- } else {
- /*
- * It's not in waiting_for_cells, so it can't become pending; it's
- * either not in any of the lists (we add it to waiting_to_write)
- * or it's already in waiting_to_write or pending (we do nothing)
- */
- if (!(smartlist_contains(channels_waiting_to_write, chan) ||
- smartlist_contains(channels_pending, chan))) {
- smartlist_add(channels_waiting_to_write, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p entered waiting_to_write",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- }
- }
- /*
- * If we made a channel pending, we potentially have scheduling work
- * to do.
- */
- if (became_pending) scheduler_retrigger();
- }
- /** Set up the scheduling system */
- void
- scheduler_init(void)
- {
- log_debug(LD_SCHED, "Initting scheduler");
- tor_assert(!run_sched_ev);
- run_sched_ev = tor_event_new(tor_libevent_get_base(), -1,
- 0, scheduler_evt_callback, NULL);
- channels_waiting_for_cells = smartlist_new();
- channels_waiting_to_write = smartlist_new();
- channels_pending = smartlist_new();
- }
- /** Check if there's more scheduling work */
- static int
- scheduler_more_work(void)
- {
- tor_assert(channels_pending);
- return (smartlist_len(channels_pending) > 0) ? 1 : 0;
- }
- /** Retrigger the scheduler in a way safe to use from the callback */
- static void
- scheduler_retrigger(void)
- {
- tor_assert(run_sched_ev);
- if (!evtimer_pending(run_sched_ev, NULL)) {
- log_debug(LD_SCHED, "Retriggering scheduler event");
- event_del(run_sched_ev);
- evtimer_add(run_sched_ev, &run_sched_tv);
- }
- }
- /** Notify the scheduler of a channel being closed */
- void
- scheduler_release_channel(channel_t *chan)
- {
- tor_assert(chan);
- tor_assert(channels_waiting_for_cells);
- tor_assert(channels_waiting_to_write);
- tor_assert(channels_pending);
- smartlist_remove(channels_waiting_for_cells, chan);
- smartlist_remove(channels_waiting_to_write, chan);
- smartlist_remove(channels_pending, chan);
- }
- /** Run the scheduling algorithm if necessary */
- void
- scheduler_run(void)
- {
- smartlist_t *tmp = NULL;
- log_debug(LD_SCHED, "We have a chance to run the scheduler");
- /*
- * TODO make this work properly
- *
- * For now, just empty the pending list and log that we saw stuff in it
- */
- tmp = channels_pending;
- channels_pending = smartlist_new();
- SMARTLIST_FOREACH_BEGIN(tmp, channel_t *, chan) {
- log_debug(LD_SCHED,
- "Scheduler saw pending channel " U64_FORMAT " at %p",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- } SMARTLIST_FOREACH_END(chan);
- smartlist_free(tmp);
- }
- /** Trigger the scheduling event so we run the scheduler later */
- static void
- scheduler_trigger(void)
- {
- log_debug(LD_SCHED, "Triggering scheduler event");
- tor_assert(run_sched_ev);
- run_sched_tv.tv_sec = 0;
- run_sched_tv.tv_usec = 0;
- evtimer_add(run_sched_ev, &run_sched_tv);
- }
- /** Mark a channel as ready to accept writes */
- void
- scheduler_channel_wants_writes(channel_t *chan)
- {
- int became_pending = 0;
- tor_assert(chan);
- tor_assert(channels_waiting_for_cells);
- tor_assert(channels_waiting_to_write);
- tor_assert(channels_pending);
- /* If it's already in waiting_to_write, we can put it in pending */
- if (smartlist_contains(channels_waiting_to_write, chan)) {
- /*
- * It's in channels_waiting_to_write, so it shouldn't be in any of
- * the other lists. It can write now, so it goes to channels_pending.
- */
- smartlist_remove(channels_waiting_to_write, chan);
- smartlist_add(channels_pending, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p went from waiting_to_write "
- "to pending",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- became_pending = 1;
- } else {
- /*
- * It's not in waiting_to_write, so it can't become pending; it's
- * either not in any of the lists (we add it to waiting_for_cells)
- * or it's already in waiting_for_cells or pending (we do nothing)
- */
- if (!(smartlist_contains(channels_waiting_for_cells, chan) ||
- smartlist_contains(channels_pending, chan))) {
- smartlist_add(channels_waiting_for_cells, chan);
- log_debug(LD_SCHED,
- "Channel " U64_FORMAT " at %p entered waiting_for_cells",
- U64_PRINTF_ARG(chan->global_identifier), chan);
- }
- }
- /*
- * If we made a channel pending, we potentially have scheduling work
- * to do.
- */
- if (became_pending) scheduler_retrigger();
- }
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