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

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Message-Id: <15FC1715-D77C-4E31-811B-60898C0C6DAA@linaro.org>
Date:   Wed, 25 Jan 2017 09:46:29 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...com>
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


> 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?

Thanks,
Paolo

> I'm assuming that is it, let me know.
> 
> -- 
> Jens Axboe
>