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

[Fabiano Rosas <farosas@suse.de>]

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.

>   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?

>
> 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()

> +             * 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;