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

[Peter Xu <peterx@redhat.com>]

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?

> 
> >   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?

(maybe I can repost this single patch in-place to avoid another round of
 mail bombs..)

-- 
Peter Xu