Update workqueue implementation to use a single queue for the work

Previously I used one queue per worker; now I use one queue for
everyone.  The "broadcast" code is gone, replaced with an idempotent
'update' operation.

src/common/compat_pthreads.c

@@ -5,6 +5,7 @@
 
 #include "orconfig.h"
 #include <pthread.h>
+#include <signal.h>
 
 #include "compat.h"
 #include "torlog.h"

src/common/workqueue.c

@@ -1,4 +1,4 @@
-/* Copyright (c) 2013, The Tor Project, Inc. */
+/* Copyright (c) 2013-2015, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
 #include "orconfig.h"
@@ -13,8 +13,23 @@ struct threadpool_s {
   /** An array of pointers to workerthread_t: one for each running worker
    * thread. */
   struct workerthread_s **threads;
-  /** Index of the next thread that we'll give work to.*/
-  int next_for_work;
+
+  /** Condition variable that we wait on when we have no work, and which
+   * gets signaled when our queue becomes nonempty. */
+  tor_cond_t condition;
+  /** Queue of pending work that we have to do. */
+  TOR_TAILQ_HEAD(, workqueue_entry_s) work;
+
+  /** The current 'update generation' of the threadpool.  Any thread that is
+   * at an earlier generation needs to run the update function. */
+  unsigned generation;
+
+  /** Function that should be run for updates on each thread. */
+  int (*update_fn)(void *, void *);
+  /** Function to free update arguments if they can't be run. */
+  void (*free_update_arg_fn)(void *);
+  /** Array of n_threads update arguments. */
+  void **update_args;
 
   /** Number of elements in threads. */
   int n_threads;
@@ -34,10 +49,10 @@ struct workqueue_entry_s {
   /** The next workqueue_entry_t that's pending on the same thread or
    * reply queue. */
   TOR_TAILQ_ENTRY(workqueue_entry_s) next_work;
-  /** The thread to which this workqueue_entry_t was assigned. This field
+  /** The threadpool to which this workqueue_entry_t was assigned. This field
    * is set when the workqueue_entry_t is created, and won't be cleared until
    * after it's handled in the main thread. */
-  struct workerthread_s *on_thread;
+  struct threadpool_s *on_pool;
   /** True iff this entry is waiting for a worker to start processing it. */
   uint8_t pending;
   /** Function to run in the worker thread. */
@@ -62,13 +77,10 @@ struct replyqueue_s {
  * contention, each gets its own queue. This breaks the guarantee that that
  * queued work will get executed strictly in order. */
 typedef struct workerthread_s {
-  /** Lock to protect all fields of this thread and its queue. */
-  tor_mutex_t lock;
-  /** Condition variable that we wait on when we have no work, and which
-   * gets signaled when our queue becomes nonempty. */
-  tor_cond_t condition;
-  /** Queue of pending work that we have to do. */
-  TOR_TAILQ_HEAD(, workqueue_entry_s) work;
+  /** Which thread it this?  In range 0..in_pool->n_threads-1 */
+  int index;
+  /** The pool this thread is a part of. */
+  struct threadpool_s *in_pool;
   /** True iff this thread is currently in its loop. (Not currently used.) */
   unsigned is_running;
   /** True iff this thread has crashed or is shut down for some reason. (Not
@@ -81,6 +93,8 @@ typedef struct workerthread_s {
   void *state;
   /** Reply queue to which we pass our results. */
   replyqueue_t *reply_queue;
+  /** The current update generation of this thread */
+  unsigned generation;
 } workerthread_t;
 
 static void queue_reply(replyqueue_t *queue, workqueue_entry_t *work);
@@ -132,13 +146,13 @@ workqueue_entry_cancel(workqueue_entry_t *ent)
 {
   int cancelled = 0;
   void *result = NULL;
-  tor_mutex_acquire(&ent->on_thread->lock);
+  tor_mutex_acquire(&ent->on_pool->lock);
   if (ent->pending) {
-    TOR_TAILQ_REMOVE(&ent->on_thread->work, ent, next_work);
+    TOR_TAILQ_REMOVE(&ent->on_pool->work, ent, next_work);
     cancelled = 1;
     result = ent->arg;
   }
-  tor_mutex_release(&ent->on_thread->lock);
+  tor_mutex_release(&ent->on_pool->lock);
 
   if (cancelled) {
     tor_free(ent);
@@ -146,6 +160,16 @@ workqueue_entry_cancel(workqueue_entry_t *ent)
   return result;
 }
 
+/**DOCDOC
+
+   must hold lock */
+static int
+worker_thread_has_work(workerthread_t *thread)
+{
+  return !TOR_TAILQ_EMPTY(&thread->in_pool->work) ||
+    thread->generation != thread->in_pool->generation;
+}
+
 /**
  * Main function for the worker thread.
  */
@@ -153,20 +177,39 @@ static void
 worker_thread_main(void *thread_)
 {
   workerthread_t *thread = thread_;
+  threadpool_t *pool = thread->in_pool;
   workqueue_entry_t *work;
   int result;
 
-  tor_mutex_acquire(&thread->lock);
   thread->is_running = 1;
+
+  tor_mutex_acquire(&pool->lock);
   while (1) {
     /* lock must be held at this point. */
-    while (!TOR_TAILQ_EMPTY(&thread->work)) {
+    while (worker_thread_has_work(thread)) {
       /* lock must be held at this point. */
-
-      work = TOR_TAILQ_FIRST(&thread->work);
-      TOR_TAILQ_REMOVE(&thread->work, work, next_work);
+      if (thread->in_pool->generation != thread->generation) {
+        void *arg = thread->in_pool->update_args[thread->index];
+        thread->in_pool->update_args[thread->index] = NULL;
+        int (*update_fn)(void*,void*) = thread->in_pool->update_fn;
+        thread->generation = thread->in_pool->generation;
+        tor_mutex_release(&pool->lock);
+
+        int r = update_fn(thread->state, arg);
+
+        if (r < 0) {
+          thread->is_running = 0;
+          thread->is_shut_down = 1;
+          return;
+        }
+
+        tor_mutex_acquire(&pool->lock);
+        continue;
+      }
+      work = TOR_TAILQ_FIRST(&pool->work);
+      TOR_TAILQ_REMOVE(&pool->work, work, next_work);
       work->pending = 0;
-      tor_mutex_release(&thread->lock);
+      tor_mutex_release(&pool->lock);
 
       /* We run the work function without holding the thread lock. This
        * is the main thread's first opportunity to give us more work. */
@@ -175,25 +218,23 @@ worker_thread_main(void *thread_)
       /* Queue the reply for the main thread. */
       queue_reply(thread->reply_queue, work);
 
-      tor_mutex_acquire(&thread->lock);
       /* We may need to exit the thread. */
       if (result >= WQ_RPL_ERROR) {
         thread->is_running = 0;
         thread->is_shut_down = 1;
-        tor_mutex_release(&thread->lock);
         return;
       }
+      tor_mutex_acquire(&pool->lock);
     }
     /* At this point the lock is held, and there is no work in this thread's
      * queue. */
 
-    /* TODO: Try work-stealing. */
     /* TODO: support an idle-function */
 
     /* Okay. Now, wait till somebody has work for us. */
     /* XXXX we could just omit waiting and instead */
     thread->waiting = 1;
-    if (tor_cond_wait(&thread->condition, &thread->lock, NULL) < 0) {
+    if (tor_cond_wait(&pool->condition, &pool->lock, NULL) < 0) {
       /* XXXX ERROR */
     }
     thread->waiting = 0;
@@ -221,14 +262,12 @@ queue_reply(replyqueue_t *queue, workqueue_entry_t *work)
 /** Allocate and start a new worker thread to use state object <b>state</b>,
  * and send responses to <b>replyqueue</b>. */
 static workerthread_t *
-workerthread_new(void *state, replyqueue_t *replyqueue)
+workerthread_new(void *state, threadpool_t *pool, replyqueue_t *replyqueue)
 {
   workerthread_t *thr = tor_malloc_zero(sizeof(workerthread_t));
-  tor_mutex_init_for_cond(&thr->lock);
-  tor_cond_init(&thr->condition);
-  TOR_TAILQ_INIT(&thr->work);
   thr->state = state;
   thr->reply_queue = replyqueue;
+  thr->in_pool = pool;
 
   if (spawn_func(worker_thread_main, thr) < 0) {
     log_err(LD_GENERAL, "Can't launch worker thread.");
@@ -238,30 +277,6 @@ workerthread_new(void *state, replyqueue_t *replyqueue)
   return thr;
 }
 
-/**
- * Add an item of work to a single worker thread. See threadpool_queue_work(*)
- * for arguments.
- */
-static workqueue_entry_t *
-workerthread_queue_work(workerthread_t *worker,
-                        int (*fn)(void *, void *),
-                        void (*reply_fn)(void *),
-                        void *arg)
-{
-  workqueue_entry_t *ent = workqueue_entry_new(fn, reply_fn, arg);
-
-  tor_mutex_acquire(&worker->lock);
-  ent->on_thread = worker;
-  ent->pending = 1;
-  TOR_TAILQ_INSERT_TAIL(&worker->work, ent, next_work);
-
-  if (worker->waiting) /* XXXX inside or outside of lock?? */
-    tor_cond_signal_one(&worker->condition);
-
-  tor_mutex_release(&worker->lock);
-  return ent;
-}
-
 /**
  * Queue an item of work for a thread in a thread pool.  The function
  * <b>fn</b> will be run in a worker thread, and will receive as arguments the
@@ -285,20 +300,19 @@ threadpool_queue_work(threadpool_t *pool,
                       void (*reply_fn)(void *),
                       void *arg)
 {
-  workerthread_t *worker;
+  workqueue_entry_t *ent = workqueue_entry_new(fn, reply_fn, arg);
+  ent->on_pool = pool;
+  ent->pending = 1;
 
   tor_mutex_acquire(&pool->lock);
-  /* Pick the next thread in random-access order. */
-  worker = pool->threads[pool->next_for_work++];
-  if (!worker) {
-    tor_mutex_release(&pool->lock);
-    return NULL;
-  }
-  if (pool->next_for_work >= pool->n_threads)
-    pool->next_for_work = 0;
+
+  TOR_TAILQ_INSERT_TAIL(&pool->work, ent, next_work);
+
   tor_mutex_release(&pool->lock);
 
-  return workerthread_queue_work(worker, fn, reply_fn, arg);
+  tor_cond_signal_one(&pool->condition);
+
+  return ent;
 }
 
 /**
@@ -309,30 +323,56 @@ threadpool_queue_work(threadpool_t *pool,
  * <b>arg</b> value is passed to <b>dup_fn</b> once per each thread to
  * make a copy of it.
  *
+ * UPDATE FUNCTIONS MUST BE IDEMPOTENT.  We do not guarantee that every update
+ * will be run.  If a new update is scheduled before the old update finishes
+ * running, then the new will replace the old in any threads that haven't run
+ * it yet.
+ *
  * Return 0 on success, -1 on failure.
  */
 int
-threadpool_queue_for_all(threadpool_t *pool,
+threadpool_queue_update(threadpool_t *pool,
                          void *(*dup_fn)(void *),
                          int (*fn)(void *, void *),
-                         void (*reply_fn)(void *),
+                         void (*free_fn)(void *),
                          void *arg)
 {
-  int i = 0;
-  workerthread_t *worker;
-  void *arg_copy;
-  while (1) {
-    tor_mutex_acquire(&pool->lock);
-    if (i >= pool->n_threads) {
-      tor_mutex_release(&pool->lock);
-      return 0;
-    }
-    worker = pool->threads[i++];
-    tor_mutex_release(&pool->lock);
+  int i, n_threads;
+  void (*old_args_free_fn)(void *arg);
+  void **old_args;
+  void **new_args;
+
+  tor_mutex_acquire(&pool->lock);
+  n_threads = pool->n_threads;
+  old_args = pool->update_args;
+  old_args_free_fn = pool->free_update_arg_fn;
+
+  new_args = tor_calloc(n_threads, sizeof(void*));
+  for (i = 0; i < n_threads; ++i) {
+    if (dup_fn)
+      new_args[i] = dup_fn(arg);
+    else
+      new_args[i] = arg;
+  }
+
+  pool->update_args = new_args;
+  pool->free_update_arg_fn = free_fn;
+  pool->update_fn = fn;
+  ++pool->generation;
 
-    arg_copy = dup_fn ? dup_fn(arg) : arg;
-    /* CHECK*/ workerthread_queue_work(worker, fn, reply_fn, arg_copy);
+  tor_mutex_release(&pool->lock);
+
+  tor_cond_signal_all(&pool->condition);
+
+  if (old_args) {
+    for (i = 0; i < n_threads; ++i) {
+      if (old_args[i] && old_args_free_fn)
+        old_args_free_fn(old_args[i]);
+    }
+    tor_free(old_args);
   }
+
+  return 0;
 }
 
 /** Launch threads until we have <b>n</b>. */
@@ -346,7 +386,8 @@ threadpool_start_threads(threadpool_t *pool, int n)
 
   while (pool->n_threads < n) {
     void *state = pool->new_thread_state_fn(pool->new_thread_state_arg);
-    workerthread_t *thr = workerthread_new(state, pool->reply_queue);
+    workerthread_t *thr = workerthread_new(state, pool, pool->reply_queue);
+    thr->index = pool->n_threads;
 
     if (!thr) {
       tor_mutex_release(&pool->lock);
@@ -375,7 +416,10 @@ threadpool_new(int n_threads,
 {
   threadpool_t *pool;
   pool = tor_malloc_zero(sizeof(threadpool_t));
-  tor_mutex_init(&pool->lock);
+  tor_mutex_init_nonrecursive(&pool->lock);
+  tor_cond_init(&pool->condition);
+  TOR_TAILQ_INIT(&pool->work);
+
   pool->new_thread_state_fn = new_thread_state_fn;
   pool->new_thread_state_arg = arg;
   pool->free_thread_state_fn = free_thread_state_fn;
@@ -447,7 +491,7 @@ replyqueue_process(replyqueue_t *queue)
     workqueue_entry_t *work = TOR_TAILQ_FIRST(&queue->answers);
     TOR_TAILQ_REMOVE(&queue->answers, work, next_work);
     tor_mutex_release(&queue->lock);
-    work->on_thread = NULL;
+    work->on_pool = NULL;
 
     work->reply_fn(work->arg);
     workqueue_entry_free(work);

src/common/workqueue.h

@@ -27,11 +27,11 @@ workqueue_entry_t *threadpool_queue_work(threadpool_t *pool,
                                          int (*fn)(void *, void *),
                                          void (*reply_fn)(void *),
                                          void *arg);
-int threadpool_queue_for_all(threadpool_t *pool,
-                             void *(*dup_fn)(void *),
-                             int (*fn)(void *, void *),
-                             void (*reply_fn)(void *),
-                             void *arg);
+int threadpool_queue_update(threadpool_t *pool,
+                            void *(*dup_fn)(void *),
+                            int (*fn)(void *, void *),
+                            void (*free_fn)(void *),
+                            void *arg);
 void *workqueue_entry_cancel(workqueue_entry_t *pending_work);
 threadpool_t *threadpool_new(int n_threads,
                              replyqueue_t *replyqueue,

src/or/cpuworker.c

@@ -170,11 +170,6 @@ update_state_threadfn(void *state_, void *work_)
   ++state->generation;
   return WQ_RPL_REPLY;
 }
-static void
-update_state_replyfn(void *work_)
-{
-  tor_free(work_);
-}
 
 /** Called when the onion key has changed and we need to spawn new
  * cpuworkers.  Close all currently idle cpuworkers, and mark the last
@@ -183,11 +178,11 @@ update_state_replyfn(void *work_)
 void
 cpuworkers_rotate_keyinfo(void)
 {
-  if (threadpool_queue_for_all(threadpool,
-                               worker_state_new,
-                               update_state_threadfn,
-                               update_state_replyfn,
-                               NULL)) {
+  if (threadpool_queue_update(threadpool,
+                              worker_state_new,
+                              update_state_threadfn,
+                              worker_state_free,
+                              NULL)) {
     log_warn(LD_OR, "Failed to queue key update for worker threads.");
   }
 }

src/test/test_workqueue.c

@@ -132,7 +132,6 @@ new_state(void *arg)
   /* Every thread gets its own keys. not a problem for benchmarking */
   st->rsa = crypto_pk_new();
   if (crypto_pk_generate_key_with_bits(st->rsa, 1024) < 0) {
-    puts("keygen failed");
     crypto_pk_free(st->rsa);
     tor_free(st);
     return NULL;
@@ -213,7 +212,6 @@ add_n_work_items(threadpool_t *tp, int n)
   while (n_queued++ < n) {
     ent = add_work(tp);
     if (! ent) {
-      puts("Couldn't add work.");
       tor_event_base_loopexit(tor_libevent_get_base(), NULL);
       return -1;
     }
@@ -238,18 +236,6 @@ add_n_work_items(threadpool_t *tp, int n)
 }
 
 static int shutting_down = 0;
-static int n_shutdowns_done = 0;
-
-static void
-shutdown_reply(void *arg)
-{
-  (void)arg;
-  tor_assert(shutting_down);
-  ++n_shutdowns_done;
-  if (n_shutdowns_done == opt_n_threads) {
-    tor_event_base_loopexit(tor_libevent_get_base(), NULL);
-  }
-}
 
 static void
 replysock_readable_cb(tor_socket_t sock, short what, void *arg)
@@ -297,8 +283,8 @@ replysock_readable_cb(tor_socket_t sock, short what, void *arg)
       n_received+n_successful_cancel == n_sent &&
       n_sent >= opt_n_items) {
     shutting_down = 1;
-    threadpool_queue_for_all(tp, NULL,
-                             workqueue_do_shutdown, shutdown_reply, NULL);
+    threadpool_queue_update(tp, NULL,
+                             workqueue_do_shutdown, NULL, NULL);
   }
 }
 
@@ -410,8 +396,9 @@ main(int argc, char **argv)
 
   event_base_loop(tor_libevent_get_base(), 0);
 
-  if (n_sent != opt_n_items || n_received+n_successful_cancel != n_sent ||
-      n_shutdowns_done != opt_n_threads) {
+  if (n_sent != opt_n_items || n_received+n_successful_cancel != n_sent) {
+    printf("%d vs %d\n", n_sent, opt_n_items);
+    printf("%d+%d vs %d\n", n_received, n_successful_cancel, n_sent);
     puts("FAIL");
     return 1;
   } else {