Re: [PATCH 07/14] migration/multifd: Simplify locking in sender thread

[Fabiano Rosas <farosas@suse.de>]

peterx@redhat.com writes:

> From: Peter Xu <peterx@redhat.com>
>
> The sender thread will yield the p->mutex before IO starts, trying to not
> block the requester thread.  This may be unnecessary lock optimizations,
> because the requester can already read pending_job safely even without the
> lock, because the requester is currently the only one who can assign a
> task.

What about the coroutine yield at qio_channel_writev_full_all()? Is it
safe from yield while holding a lock? Could the main loop dispatch the
cleanup function, it calls join on the multifd thread and it deadlocks?

>
> Drop that lock complication on both sides:
>
>   (1) in the sender thread, always take the mutex until job done
>   (2) in the requester thread, check pending_job clear lockless
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/multifd.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 6a4863edd2..4dc5af0a15 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -429,7 +429,9 @@ static int multifd_send_pages(void)
>          return -1;
>      }
>  
> +    /* We wait here, until at least one channel is ready */
>      qemu_sem_wait(&multifd_send_state->channels_ready);
> +
>      /*
>       * next_channel can remain from a previous migration that was
>       * using more channels, so ensure it doesn't overflow if the
> @@ -441,17 +443,26 @@ static int multifd_send_pages(void)
>              return -1;
>          }
>          p = &multifd_send_state->params[i];
> -        qemu_mutex_lock(&p->mutex);
> +        /*
> +         * Lockless read to p->pending_job is safe, because only multifd
> +         * sender thread can clear it.
> +         */
>          if (!p->pending_job) {

The worst it could happen is we read at the same time the thread is
clearing it and we loop to the next channel. So it doesn't need to be
atomic either.

> -            p->pending_job = true;
>              next_channel = (i + 1) % migrate_multifd_channels();
>              break;
>          }
> -        qemu_mutex_unlock(&p->mutex);
>      }
> +
> +    qemu_mutex_lock(&p->mutex);

What data this lock protects now? Everything below here only happens
after this thread sees pending_job==false. It seems we would only need a
barrier on the multifd thread to make sure p->pending_job=false is
ordered after everything.

Even for the "sync" case, it appears the lock is not needed as well?

We might need to remove p->running first and move the kick from
multifd_send_terminate_threads() into multifd_save_cleanup() like I
suggested, but it seems like we could remove this lock.

Which would make sense, because there's nothing another thread would
want to do with a channel's MultiFDSendParams unless the channel is idle
waiting for work.

> +    /*
> +     * 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.
> +     */
> +    assert(p->pending_job == false);
> +    p->pending_job = true;
>      assert(!p->pages->num);
>      assert(!p->pages->block);
> -
>      p->packet_num = multifd_send_state->packet_num++;

I noticed this line cannot be here. If the channel thread takes long to
wakeup, the "sync" code will increment once more and overwrite this
field. This and the identical line at multifd_send_sync_main() should go
into multifd_send_fill_packet() I think.

>      multifd_send_state->pages = p->pages;
>      p->pages = pages;
> @@ -704,8 +715,6 @@ static void *multifd_send_thread(void *opaque)
>              multifd_send_fill_packet(p);
>              p->num_packets++;
>              p->total_normal_pages += pages->num;
> -            qemu_mutex_unlock(&p->mutex);
> -
>              trace_multifd_send(p->id, packet_num, pages->num, p->flags,
>                                 p->next_packet_size);
>  
> @@ -725,6 +734,7 @@ static void *multifd_send_thread(void *opaque)
>              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;
>              }
>  
> @@ -733,7 +743,6 @@ static void *multifd_send_thread(void *opaque)
>  
>              multifd_pages_reset(p->pages);
>              p->next_packet_size = 0;
> -            qemu_mutex_lock(&p->mutex);
>              p->pending_job = false;
>              qemu_mutex_unlock(&p->mutex);
>          } else if (p->pending_sync) {