Re: [PATCH] sbitmap: Advance the queue index before waking up the queue

Jan Kara
2022-11-14T14:34:12Z

On Mon 14-11-22 09:20:57, Gabriel Krisman Bertazi wrote:
> Jan Kara <jack@suse.cz> writes:
>
> > Gabriel, when looking through this patch, I've noticed we can loose wakeups
> > after your latest simplifications. See below for details:
> >
> > On Sat 05-11-22 19:10:55, Gabriel Krisman Bertazi wrote:
> >> @@ -587,7 +571,7 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> >> for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> >> struct sbq_wait_state *ws = &sbq->ws[wake_index];
> >>
> >> - if (waitqueue_active(&ws->wait) && atomic_read(&ws->wait_cnt)) {
> >> + if (waitqueue_active(&ws->wait)) {
> >> if (wake_index != atomic_read(&sbq->wake_index))
> >> atomic_set(&sbq->wake_index, wake_index);
> >> return ws;
> >
> > Neither sbq_wake_ptr() nor sbitmap_queue_wake_up() now increment the
> > wake_index after performing the wakeup. Thus we would effectively keep
> > waking tasks from a single waitqueue until it becomes empty and only then
> > go for the next waitqueue. This creates unnecessary unfairness in task
> > wakeups and even possible starvation issues. So I think we need to advance
> > wake_index somewhere. Perhaps here before returning waitqueue.
>
> right. This is indeed a problem. what do you think of the patch below?
>
> > Now this may be also problematic - when we were checking the number of woken
> > waiters in the older version of the patch (for others: internal version of
> > the patch) this was fine but now it may happen that the 'ws' we have
> > selected has no waiters anymore. And in that case we need to find another
> > waitqueue because otherwise we'd be loosing too many wakeups and we could
> > deadlock. So I think this rather needs to be something like:
> >
> > do {
> > if (atomic_read(&sbq->completion_cnt) - wakeups < wake_batch)
> > return;
> > } while (!atomic_try_cmpxchg(&sbq->wakeup_cnt,
> > &wakeups, wakeups + wake_batch));
> >
> > do {
> > ws = sbq_wake_ptr(sbq);
> > if (!ws)
> > return;
> > } while (!wake_up_nr(&ws->wait, wake_batch));
> >
> > with our original version of wake_up_nr() returning number of woken
> > waiters. What do you think?
>
> I agree, and it wouldn't happen with the wake_up_nr patch we had before.
> I will revive it quickly and follow up. But, in this case, I want to be
> cautious with benchmarking, so I will follow up still today, but as soon
> as the new round of tests complete.

Sure.

> -- >8 --
> Subject: [PATCH] sbitmap: Advance the queue index before waking up the queue
>
> When a queue is awaken, the wake_index written by sbq_wake_ptr currently
> keeps pointing to the same queue. On the next wake up, it will thus
> retry the same queue, which is unfair to other queues, and can lead to
> starvation. This patch, moves the index update to happen before the
> queue is returned, such that it will now try a different queue first on
> the next wake up, improving fairness.
>
> Reported-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>

Yes, nice. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> lib/sbitmap.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index eca462cba398..bea7984f7987 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -571,13 +571,19 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> struct sbq_wait_state *ws = &sbq->ws[wake_index];
>
> + /*
> + * Advance the index before checking the current queue.
> + * It improves fairness, by ensuring the queue doesn't
> + * need to be fully emptied before trying to wake up
> + * from the next one.
> + */
> + wake_index = sbq_index_inc(wake_index);
> +
> if (waitqueue_active(&ws->wait)) {
> if (wake_index != atomic_read(&sbq->wake_index))
> atomic_set(&sbq->wake_index, wake_index);
> return ws;
> }
> -
> - wake_index = sbq_index_inc(wake_index);
> }
>
> return NULL;
> --
> 2.35.3
>
>
>
>
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR