Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Subject: Re: [PATCH 1/5] sbitmap: don't consume nr for inactive waitqueue to
 avoid lost wakeups
To:     Gabriel Krisman Bertazi <krisman@suse.de>,
        Kemeng Shi <shikemeng@huawei.com>
Cc:     axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linfeilong@huawei.com,
        liuzhiqiang@huawei.com
References: <20221201045408.21908-1-shikemeng@huawei.com>
 <20221201045408.21908-2-shikemeng@huawei.com> <87y1rrmeq3.fsf@suse.de>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <20551512-a703-9637-29d8-931f5a76e392@huaweicloud.com>
Date:   Fri, 2 Dec 2022 08:57:20 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.5.0
MIME-Version: 1.0
In-Reply-To: <87y1rrmeq3.fsf@suse.de>
Content-Type: text/plain; charset=gbk
Content-Transfer-Encoding: 7bit
Precedence: bulk


on 12/1/2022 9:32 PM, Gabriel Krisman Bertazi wrote:
> Kemeng Shi <shikemeng@huawei.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