Peter Xu <pet...@redhat.com> writes: > On Mon, Feb 05, 2024 at 11:10:34AM -0300, Fabiano Rosas wrote: >> > (maybe I can repost this single patch in-place to avoid another round of >> > mail bombs..) >> >> Sure. > > I've got the final version attached here. Feel free to have a look, thanks. > > ==== > From 6ba337320430feae4ce9d3d906ea19f68430642d Mon Sep 17 00:00:00 2001 > From: Peter Xu <pet...@redhat.com> > Date: Fri, 2 Feb 2024 18:28:57 +0800 > Subject: [PATCH] migration/multifd: Optimize sender side to be lockless > > 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> > 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
Reviewed-by: Fabiano Rosas <faro...@suse.de>
