migration: Wake rate limiting for urgent requests

Rate limiting sleeps the migration thread for a while when it runs
out of bandwidth; but sometimes we want to wake up to get on with
something more urgent (like a postcopy request).  Here we use
a semaphore with a timedwait instead of a simple sleep; Incrementing
the sempahore will wake it up sooner.  Anything that consumes
these urgent events must decrement the sempahore.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180613102642.23995-3-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2852,6 +2852,16 @@ static void migration_iteration_finish(MigrationState *s)
     qemu_mutex_unlock_iothread();
 }
 
+void migration_make_urgent_request(void)
+{
+    qemu_sem_post(&migrate_get_current()->rate_limit_sem);
+}
+
+void migration_consume_urgent_request(void)
+{
+    qemu_sem_wait(&migrate_get_current()->rate_limit_sem);
+}
+
 /*
  * Master migration thread on the source VM.
  * It drives the migration and pumps the data down the outgoing channel.
@@ -2861,6 +2871,7 @@ static void *migration_thread(void *opaque)
     MigrationState *s = opaque;
     int64_t setup_start = qemu_clock_get_ms(QEMU_CLOCK_HOST);
     MigThrError thr_error;
+    bool urgent = false;
 
     rcu_register_thread();
 
@@ -2901,7 +2912,7 @@ static void *migration_thread(void *opaque)
            s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
         int64_t current_time;
 
-        if (!qemu_file_rate_limit(s->to_dst_file)) {
+        if (urgent || !qemu_file_rate_limit(s->to_dst_file)) {
             MigIterateState iter_state = migration_iteration_run(s);
             if (iter_state == MIG_ITERATE_SKIP) {
                 continue;
@@ -2932,10 +2943,24 @@ static void *migration_thread(void *opaque)
 
         migration_update_counters(s, current_time);
 
+        urgent = false;
         if (qemu_file_rate_limit(s->to_dst_file)) {
-            /* usleep expects microseconds */
-            g_usleep((s->iteration_start_time + BUFFER_DELAY -
-                      current_time) * 1000);
+            /* Wait for a delay to do rate limiting OR
+             * something urgent to post the semaphore.
+             */
+            int ms = s->iteration_start_time + BUFFER_DELAY - current_time;
+            trace_migration_thread_ratelimit_pre(ms);
+            if (qemu_sem_timedwait(&s->rate_limit_sem, ms) == 0) {
+                /* We were worken by one or more urgent things but
+                 * the timedwait will have consumed one of them.
+                 * The service routine for the urgent wake will dec
+                 * the semaphore itself for each item it consumes,
+                 * so add this one we just eat back.
+                 */
+                qemu_sem_post(&s->rate_limit_sem);
+                urgent = true;
+            }
+            trace_migration_thread_ratelimit_post(urgent);
         }
     }
 
@@ -3109,6 +3134,7 @@ static void migration_instance_finalize(Object *obj)
     qemu_mutex_destroy(&ms->qemu_file_lock);
     g_free(params->tls_hostname);
     g_free(params->tls_creds);
+    qemu_sem_destroy(&ms->rate_limit_sem);
     qemu_sem_destroy(&ms->pause_sem);
     qemu_sem_destroy(&ms->postcopy_pause_sem);
     qemu_sem_destroy(&ms->postcopy_pause_rp_sem);
@@ -3147,6 +3173,7 @@ static void migration_instance_init(Object *obj)
     qemu_sem_init(&ms->postcopy_pause_sem, 0);
     qemu_sem_init(&ms->postcopy_pause_rp_sem, 0);
     qemu_sem_init(&ms->rp_state.rp_sem, 0);
+    qemu_sem_init(&ms->rate_limit_sem, 0);
     qemu_mutex_init(&ms->qemu_file_lock);
 }
 

migration/migration.h

@@ -121,6 +121,11 @@ struct MigrationState
      */
     QemuMutex qemu_file_lock;
 
+    /*
+     * Used to allow urgent requests to override rate limiting.
+     */
+    QemuSemaphore rate_limit_sem;
+
     /* bytes already send at the beggining of current interation */
     uint64_t iteration_initial_bytes;
     /* time at the start of current iteration */
@@ -287,4 +292,7 @@ void init_dirty_bitmap_incoming_migration(void);
 #define qemu_ram_foreach_block \
   #warning "Use qemu_ram_foreach_block_migratable in migration code"
 
+void migration_make_urgent_request(void);
+void migration_consume_urgent_request(void);
+
 #endif

migration/trace-events

@@ -108,6 +108,8 @@ migration_return_path_end_before(void) ""
 migration_return_path_end_after(int rp_error) "%d"
 migration_thread_after_loop(void) ""
 migration_thread_file_err(void) ""
+migration_thread_ratelimit_pre(int ms) "%d ms"
+migration_thread_ratelimit_post(int urgent) "urgent: %d"
 migration_thread_setup_complete(void) ""
 open_return_path_on_source(void) ""
 open_return_path_on_source_continue(void) ""