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

[Jan Kara]

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...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR