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Subject: Re: [PATCH] sbitmap: Use single per-bitmap counting to wake up queued
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To:     Jan Kara <jack@suse.cz>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Gabriel Krisman Bertazi <krisman@suse.de>, axboe@kernel.dk,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        Hugh Dickins <hughd@google.com>,
        Keith Busch <kbusch@kernel.org>,
        Liu Song <liusong@linux.alibaba.com>,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221105231055.25953-1-krisman@suse.de>
 <2a445c5c-fd15-c0bf-8655-2fb5bde3fe67@huaweicloud.com>
 <20221110111636.ufgyp4tkbzexugk2@quack3>
 <210f2c3d-0bc1-0a5f-964b-d75020d3d9fb@huaweicloud.com>
 <20221110153533.go5qs3psm75h27mx@quack3>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <535d8806-9abb-4caf-d75b-4013a262415d@huaweicloud.com>
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
>