linux-kernel - Re: [PATCHSET v4] blk-mq-scheduling framework

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Date:   Wed, 25 Jan 2017 09:13:35 -0700
From:   Jens Axboe <axboe@...com>
To:     Paolo Valente <paolo.valente@...aro.org>
CC:     Jens Axboe <axboe@...nel.dk>, <linux-block@...r.kernel.org>,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Omar Sandoval <osandov@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCHSET v4] blk-mq-scheduling framework

On 01/25/2017 01:46 AM, Paolo Valente wrote:
> 
>> Il giorno 23 gen 2017, alle ore 18:42, Jens Axboe <axboe@...com> ha scritto:
>>
>> On 01/23/2017 10:04 AM, Paolo Valente wrote:
>>>
>>>> Il giorno 18 gen 2017, alle ore 17:21, Jens Axboe <axboe@...com> ha scritto:
>>>>
>>>> On 01/18/2017 08:14 AM, Paolo Valente wrote:
>>>>> according to the function blk_mq_sched_put_request, the
>>>>> mq.completed_request hook seems to always be invoked (if set) for a
>>>>> request for which the mq.put_rq_priv is invoked (if set).
>>>>
>>>> Correct, any request that came out of blk_mq_sched_get_request()
>>>> will always have completed called on it, regardless of whether it
>>>> had IO started on it or not.
>>>>
>>>
>>> It seems that some request, after being dispatched, happens to have no
>>> mq.put_rq_priv invoked on it now or then.  Is it expected?  If it is,
>>> could you point me to the path through which the end of the life of
>>> such a request is handled?
>>
>> I'm guessing that's a flush request. I added RQF_QUEUED to check for
>> that, if RQF_QUEUED is set, you know it has come from your get_request
>> handler.
>>
> 
> Exactly, the completion-without-put_rq_priv pattern seems to occur
> only for requests coming from the flusher, precisely because they have
> the flag RQF_ELVPRIV unset.  Just to understand: why is this flag
> unset for these requests, if they do have private elevator (bfq)
> data attached?  What am I misunderstanding?
> 
> Just to be certain: this should be the only case where the
> completed_request hook is invoked while the put_rq_priv is not, right?

They must NOT have scheduler data attached. In your get_request
function, you must bypass if blk_mq_sched_bypass_insert() returns true.
See how mq-deadline does that. This is important, or you will get hangs
with flushes as well, since the IO scheduler private data and the flush
data is unionized in the request.

-- 
Jens Axboe