From: Peter Xu <pet...@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....@suse.de Suggested-by: Fabiano Rosas <faro...@suse.de> Reviewed-by: Fabiano Rosas <faro...@suse.de> Link: https://lore.kernel.org/r/20240202102857.110210-24-pet...@redhat.com Signed-off-by: Peter Xu <pet...@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
