Re: [PATCH v2 23/23] migration/multifd: Optimize sender side to be lockless

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Fri, Feb 02, 2024 at 06:34:08PM -0300, Fabiano Rosas wrote:
>> peterx@redhat.com writes:
>> 
>> > From: Peter Xu <peterx@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.fsf@suse.de
>> >
>> > Suggested-by: Fabiano Rosas <farosas@suse.de>
>> > Signed-off-by: Peter Xu <peterx@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.