linux-kernel - Re: [PATCH 3/4] blk-mq: establish new mapping before cpu starts handling requests

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Message-ID: <CACVXFVNgVwh-6_EKenB1KKGWm51KR-fLw-oU2B+WX9TmCzkKyw@mail.gmail.com>
Date:	Thu, 25 Jun 2015 00:24:27 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 3/4] blk-mq: establish new mapping before cpu starts
 handling requests

On Wed, Jun 24, 2015 at 10:34 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> Hi Ming,
>
> 2015-06-24 18:46 GMT+09:00 Ming Lei <tom.leiming@...il.com>:
>> On Sun, Jun 21, 2015 at 9:52 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
>>> ctx->index_hw is zero for the CPUs which have never been onlined since
>>> the block queue was initialized.  If one of those CPUs is hotadded and
>>> starts handling request before new mappings are established, pending
>>
>> Could you explain a bit what the handling request is? The fact is that
>> blk_mq_queue_reinit() is run after all queues are put into freezing.
>
> Notifier callbacks for CPU_ONLINE action can be run on the other CPU
> than the CPU which was just onlined.  So it is possible for the
> process running on the just onlined CPU to insert request and run
> hw queue before blk_mq_queue_reinit_notify() is actually called with
> action=CPU_ONLINE.

You are right because blk_mq_queue_reinit_notify() is alwasy run after
the CPU becomes UP, so there is a tiny window in which the CPU is up
but the mapping is updated.  Per current design, the CPU just onlined
is still mapped to hw queue 0 until the mapping is updated by
blk_mq_queue_reinit_notify().

But I am wondering why it is a problem and why you think flush_busy_ctxs
can't find the requests on the software queue in this situation?


-- 
Ming Lei
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