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

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Date:   Thu, 26 Jan 2017 15:23:39 +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 25 gen 2017, alle ore 17:13, Jens Axboe <axboe@...com> ha scritto:
> 
> 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.

Yes, sorry.  I'm already using blk_mq_sched_bypass_insert() to bypass
insertion in the insert hook, as done in mq-deadline, and I have no
get_request defined (see below).

The source of my confusion was that I assumed that flush requests had
not to leave any trace in the scheduler, since the scheduler does not
decide anything for them.  Accordingly, I thought they did not trigger
any put or completion hook.  In contrast, these requests get the
flag QUEUED set, in case the get_request hook is set, and then trigger
both a put_request and a completed_request.  In this respect, in bfq-mq
I'm not using any of these three hooks (they are all NULL).  I hope
I'm not doing something unexpected or incoherent.

UPDATE: bfq-mq now survives for minutes.  I'm debugging two occasional
failures, which (un)fortunately become more and more occasional as I
go on with debugging and instrumenting the code.

Thanks,
Paolo

> 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