linux-kernel - Re: [PATCH 1/5] sbitmap: don't consume nr for inactive waitqueue to avoid lost wakeups

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Date:   Fri, 2 Dec 2022 08:57:20 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     Gabriel Krisman Bertazi <krisman@...e.de>,
        Kemeng Shi <shikemeng@...wei.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linfeilong@...wei.com,
        liuzhiqiang@...wei.com
Subject: Re: [PATCH 1/5] sbitmap: don't consume nr for inactive waitqueue to
 avoid lost wakeups



on 12/1/2022 9:32 PM, Gabriel Krisman Bertazi wrote:
> Kemeng Shi <shikemeng@...wei.com> writes:
> 
>> If we decremented queue without waiters, we should not decremente freed
>> bits number "nr", or all "nr" could be consumed in a empty queue and no
>> wakeup will be called.
>> Currently, for case "wait_cnt > 0", "nr" will not be decremented if we
>> decremented queue without watiers and retry is returned to avoid lost
>> wakeups. However for case "wait_cnt == 0", "nr" will be decremented
>> unconditionally and maybe decremented to zero. Although retry is
>> returned by active state of queue, it's not actually executed for "nr"
>> is zero.
>>
> 
> Hi Kemeng,
> 
> Fwiw, I sent a patch rewriting this algorithm which is now merged in
> axboe/for-next.  It drops the per-waitqueue wait_cnt entirely.  You can
> find it here:
> 
> https://lore.kernel.org/lkml/20221110153533.go5qs3psm75h27mx@quack3/T/
> 
> Thanks!
Hi Gabriel,
Thanks for remind me of this, I will recheck my patches in the
axboe/for-next branch.

-- 
Best wishes
Kemeng Shi