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;
Message-ID: <399a2c2d-0b56-e4e7-c309-a6b9537d8939@huawei.com>
Date:   Tue, 25 Oct 2022 10:32:28 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.6.1
Subject: Re: [PATCH] blk-mq: Properly init bios from
 blk_mq_alloc_request_hctx()
To:     Ming Lei <ming.lei@redhat.com>
CC:     <axboe@kernel.dk>, <linux-kernel@vger.kernel.org>,
        <linux-block@vger.kernel.org>, <hch@lst.de>,
        Bart Van Assche <bvanassche@acm.org>
References: <1666454846-11749-1-git-send-email-john.garry@huawei.com>
 <Y1U9zNZtZjRHQBww@T590> <99c6ca81-746d-85f4-04d3-49d7a3de611b@huawei.com>
 <Y1aS3vIbuQTNGWJL@T590> <360c78dc-65ce-362f-389d-075f2259ce5b@huawei.com>
 <Y1cvJ4/uwUScAQq4@T590> <3513b14c-14e0-b865-628e-a83521090de9@huawei.com>
 <CAFj5m9JnSBBVGrp5CqeH99-+VOGRuroUAi3c-3=6XKa891Sfmw@mail.gmail.com>
 <cf7f8f88-7d3e-8818-8584-e2276e7a1f30@huawei.com> <Y1epeuwonmjQhrXW@T590>
From:   John Garry <john.garry@huawei.com>
In-Reply-To: <Y1epeuwonmjQhrXW@T590>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 25/10/2022 10:16, Ming Lei wrote:
>>>> I mentioned before that if no hctx->cpumask is online then we don't need
>>>> to allocate a request. That is because if no hctx->cpumask is online,
>>>> this means that original erroneous IO must be completed due to nature of
>>>> how blk-mq cpu hotplug handler works, i.e. drained, and then we don't
>>>> actually need to abort it any longer, so ok to not get a request.
>>> No, it is really not OK, if all cpus in hctx->cpumask are offline, you
>>> can't allocate
>>> request on the specified hw queue, then the erroneous IO can't be handled,
>>> then cpu hotplug handler may hang for ever.
>> If the erroneous IO is still in-flight from blk-mq perspective, then how can
>> hctx->cpumask still be offline? I thought that we guarantee that
>> hctx->cpumask cannot go offline until drained.
> Yeah, the draining is done before the cpu is offline. But the drain is
> simply waiting for the inflight IO to be completed. If the IO is failed
> during the waiting, you can't allocate such reserved request for error
> handling, then hang ever in blk_mq_hctx_notify_offline().

Actually if final cpu in hctx->cpumask is going offline, then hctx won't 
queue any more requests, right? In this case I don't think we can queue 
on that hctx anyway. I need to think about this more.

> 
> If you just make it one driver private command, there can't be such
> issue. 

Well we're trying to use reserved requests for EH commands, which that 
goes against.

> Block layer is supposed for handling common case(normal io and pt io),
> I'd suggest to not put such special cases into block layer.

It also supports reserved commands, which I would assume would be 
suitable for EH scenarios.

Thanks,
John