From: Peter Xu <peterx@redhat.com>
When reviewing my attempt to refactor send_prepare(), Fabiano suggested we
try out with dropping the mutex in multifd code [1].
I thought about that before but I never tried to change the code. Now
maybe it's time to give it a stab. This only optimizes the sender side.
The trick here is multifd has a clear provider/consumer model, that the
migration main thread publishes requests (either pending_job/pending_sync),
while the multifd sender threads are consumers. Here we don't have a lot
of complicated data sharing, and the jobs can logically be submitted
lockless.
Arm the code with atomic weapons. Two things worth mentioning:
- For multifd_send_pages(): we can use qatomic_load_acquire() when trying
to find a free channel, but that's expensive if we attach one ACQUIRE per
channel. Instead, keep the qatomic_read() on reading the pending_job
flag as we do already, meanwhile use one smp_mb_acquire() after the loop
to guarantee the memory ordering.
- For pending_sync: it doesn't have any extra data required since now
p->flags are never touched, it should be safe to not use memory barrier.
That's different from pending_job.
Provide rich comments for all the lockless operations to state how they are
paired. With that, we can remove the mutex.
[1] https://lore.kernel.org/r/87o7d1jlu5.fsf@suse.de
Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-24-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
---
migration/multifd.h | 2 --
migration/multifd.c | 51 +++++++++++++++++++++++----------------------
2 files changed, 26 insertions(+), 27 deletions(-)
diff --git a/migration/multifd.h b/migration/multifd.h
index 98876ff94a..78a2317263 100644
--- a/migration/multifd.h
+++ b/migration/multifd.h
@@ -91,8 +91,6 @@ typedef struct {
/* syncs main thread and channels */
QemuSemaphore sem_sync;
- /* this mutex protects the following parameters */
- QemuMutex mutex;
/* is this channel thread running */
bool running;
/* multifd flags for each packet */
diff --git a/migration/multifd.c b/migration/multifd.c
index b317d57d61..fbdb129088 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -501,19 +501,19 @@ static bool multifd_send_pages(void)
}
}
- qemu_mutex_lock(&p->mutex);
- assert(!p->pages->num);
- assert(!p->pages->block);
/*
- * Double check on pending_job==false with the lock. In the future if
- * we can have >1 requester thread, we can replace this with a "goto
- * retry", but that is for later.
+ * Make sure we read p->pending_job before all the rest. Pairs with
+ * qatomic_store_release() in multifd_send_thread().
*/
- assert(qatomic_read(&p->pending_job) == false);
- qatomic_set(&p->pending_job, true);
+ smp_mb_acquire();
+ assert(!p->pages->num);
multifd_send_state->pages = p->pages;
p->pages = pages;
- qemu_mutex_unlock(&p->mutex);
+ /*
+ * Making sure p->pages is setup before marking pending_job=true. Pairs
+ * with the qatomic_load_acquire() in multifd_send_thread().
+ */
+ qatomic_store_release(&p->pending_job, true);
qemu_sem_post(&p->sem);
return true;
@@ -648,7 +648,6 @@ static bool multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
}
multifd_send_channel_destroy(p->c);
p->c = NULL;
- qemu_mutex_destroy(&p->mutex);
qemu_sem_destroy(&p->sem);
qemu_sem_destroy(&p->sem_sync);
g_free(p->name);
@@ -742,14 +741,12 @@ int multifd_send_sync_main(void)
trace_multifd_send_sync_main_signal(p->id);
- qemu_mutex_lock(&p->mutex);
/*
* We should be the only user so far, so not possible to be set by
* others concurrently.
*/
assert(qatomic_read(&p->pending_sync) == false);
qatomic_set(&p->pending_sync, true);
- qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem);
}
for (i = 0; i < migrate_multifd_channels(); i++) {
@@ -796,9 +793,12 @@ static void *multifd_send_thread(void *opaque)
if (multifd_send_should_exit()) {
break;
}
- qemu_mutex_lock(&p->mutex);
- if (qatomic_read(&p->pending_job)) {
+ /*
+ * Read pending_job flag before p->pages. Pairs with the
+ * qatomic_store_release() in multifd_send_pages().
+ */
+ if (qatomic_load_acquire(&p->pending_job)) {
MultiFDPages_t *pages = p->pages;
p->iovs_num = 0;
@@ -806,14 +806,12 @@ static void *multifd_send_thread(void *opaque)
ret = multifd_send_state->ops->send_prepare(p, &local_err);
if (ret != 0) {
- qemu_mutex_unlock(&p->mutex);
break;
}
ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num, NULL,
0, p->write_flags, &local_err);
if (ret != 0) {
- qemu_mutex_unlock(&p->mutex);
break;
}
@@ -822,24 +820,31 @@ static void *multifd_send_thread(void *opaque)
multifd_pages_reset(p->pages);
p->next_packet_size = 0;
- qatomic_set(&p->pending_job, false);
- qemu_mutex_unlock(&p->mutex);
+
+ /*
+ * Making sure p->pages is published before saying "we're
+ * free". Pairs with the smp_mb_acquire() in
+ * multifd_send_pages().
+ */
+ qatomic_store_release(&p->pending_job, false);
} else {
- /* If not a normal job, must be a sync request */
+ /*
+ * If not a normal job, must be a sync request. Note that
+ * pending_sync is a standalone flag (unlike pending_job), so
+ * it doesn't require explicit memory barriers.
+ */
assert(qatomic_read(&p->pending_sync));
p->flags = MULTIFD_FLAG_SYNC;
multifd_send_fill_packet(p);
ret = qio_channel_write_all(p->c, (void *)p->packet,
p->packet_len, &local_err);
if (ret != 0) {
- qemu_mutex_unlock(&p->mutex);
break;
}
/* p->next_packet_size will always be zero for a SYNC packet */
stat64_add(&mig_stats.multifd_bytes, p->packet_len);
p->flags = 0;
qatomic_set(&p->pending_sync, false);
- qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem_sync);
}
}
@@ -853,10 +858,7 @@ out:
error_free(local_err);
}
- qemu_mutex_lock(&p->mutex);
p->running = false;
- qemu_mutex_unlock(&p->mutex);
-
rcu_unregister_thread();
migration_threads_remove(thread);
trace_multifd_send_thread_end(p->id, p->packets_sent, p->total_normal_pages);
@@ -998,7 +1000,6 @@ int multifd_send_setup(Error **errp)
for (i = 0; i < thread_count; i++) {
MultiFDSendParams *p = &multifd_send_state->params[i];
- qemu_mutex_init(&p->mutex);
qemu_sem_init(&p->sem, 0);
qemu_sem_init(&p->sem_sync, 0);
p->id = i;
--
2.43.0