Re: [PATCH] sbitmap: Use single per-bitmap counting to wake up queued tags

Jan Kara
2022-11-10T11:16:51Z

Hi!

On Thu 10-11-22 17:42:49, Yu Kuai wrote:
> 在 2022/11/06 7:10, Gabriel Krisman Bertazi 写道:
> > +void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
> > {
> > - struct sbq_wait_state *ws;
> > - unsigned int wake_batch;
> > - int wait_cnt, cur, sub;
> > - bool ret;
> > + unsigned int wake_batch = READ_ONCE(sbq->wake_batch);
> > + struct sbq_wait_state *ws = NULL;
> > + unsigned int wakeups;
> > - if (*nr <= 0)
> > - return false;
> > + if (!atomic_read(&sbq->ws_active))
> > + return;
> > - ws = sbq_wake_ptr(sbq);
> > - if (!ws)
> > - return false;
> > + atomic_add(nr, &sbq->completion_cnt);
> > + wakeups = atomic_read(&sbq->wakeup_cnt);
> > - cur = atomic_read(&ws->wait_cnt);
> > do {
> > - /*
> > - * For concurrent callers of this, callers should call this
> > - * function again to wakeup a new batch on a different 'ws'.
> > - */
> > - if (cur == 0)
> > - return true;
> > - sub = min(*nr, cur);
> > - wait_cnt = cur - sub;
> > - } while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));
> > -
> > - /*
> > - * If we decremented queue without waiters, retry to avoid lost
> > - * wakeups.
> > - */
> > - if (wait_cnt > 0)
> > - return !waitqueue_active(&ws->wait);
> > + if (atomic_read(&sbq->completion_cnt) - wakeups < wake_batch)
> > + return;
>
> Should it be considered that completion_cnt overflow and becomes
> negtive?

Yes, the counters can (and will) certainly overflow but since we only care
about (completion_cnt - wakeups), we should be fine - this number is always
sane (and relatively small) and in the kernel we do compile with signed
overflows being well defined.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR