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migration/multifd: Optimize sender side to be lockless

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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>
This commit is contained in:
Peter Xu 2024-02-02 18:28:57 +08:00
parent 98ea497d8b
commit 488c84acb4
2 changed files with 26 additions and 27 deletions
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51
migration/multifd.c
View File

@ -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
migration/multifd.h
View File

@ -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 */