Peter Xu <pet...@redhat.com> writes: > 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? >
Yep, that's better. Thanks. >> >> > 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? Nope, that's it. > > (maybe I can repost this single patch in-place to avoid another round of > mail bombs..) Sure.
