On Fri, Feb 02, 2024 at 06:34:08PM -0300, Fabiano Rosas wrote: > pet...@redhat.com writes: > > > 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 comlicated data sharing, and the jobs can logically be submitted > > lockless. > > complicated > > > > > 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, make it atomic_read() on the pending_job flag, but > > s/make it/keep it/ > > The diff doesn't show the atomic_read already there so it's confusing.
Right. I also has a trivial typo on s/atomic_read/qatomic_read/.. I tried to rephrase the last sentence: - 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. Maybe slightly clearer? > > > merge the ACQUIRE into one single smp_mb_acquire() later. > > > > - 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_sync. > > pending_job? Yep, all the rest fixed. > > > > > 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> > > 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..ef13e2e781 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 qatomic_load_acquire() in > > smp_mb_acquire() Fixed. Any more comment on the code changes before I repost? (maybe I can repost this single patch in-place to avoid another round of mail bombs..) -- Peter Xu
