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

[Yu Kuai]

Hi

在 2022/11/10 23:35, Jan Kara 写道:
> Hi!
>
> On Thu 10-11-22 21:18:19, Yu Kuai wrote:
> > 在 2022/11/10 19:16, Jan Kara 写道:
> > > 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.
> >
> > I'm worried about this: for example, the extreme scenaro that there
> > is only one tag, currently there are only one infight rq and one thread
> > is waiting for tag. When the infight rq complete, if 'completion_cnt'
> > overflow to negative, then 'atomic_read(&sbq->completion_cnt) - wakeups
> > < wake_batch' will be passed unexpected, then will the thread never be
> > woken up if there are no new io issued ?
>
> Well but my point is that 'wakeups' is staying close to completion_cnt. So
> if completion_cnt wraps to INT_MIN, then 'wakeups' is close to INT_MAX and
> so completion_cnt - wakeups is going to wrap back and still result in a
> small number. That is simply how wrapping arithmetics works...

Yes, you're right, I'm being foolish here. 😆

Thanks,
Kuai
> 								Honza