Messages in this thread | | | Subject | Re: [PATCH] sbitmap: Use single per-bitmap counting to wake up queued tags | From | Yu Kuai <> | Date | Fri, 11 Nov 2022 08:59:01 +0800 |
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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 >
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