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

From: Paolo Valente <paolo.valente@linaro.org>
To: Jens Axboe <axboe@fb.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Linux-Kernal <linux-kernel@vger.kernel.org>,
	Omar Sandoval <osandov@fb.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [PATCHSET v4] blk-mq-scheduling framework
Date: Thu, 26 Jan 2017 15:23:39 +0100	[thread overview]
Message-ID: <CAC36074-9BB6-4A35-94EA-73ACCFD6C86E@linaro.org> (raw)
In-Reply-To: <0e047692-5ff2-d404-a400-8484e060218b@fb.com>

> Il giorno 25 gen 2017, alle ore 17:13, Jens Axboe <axboe@fb.com> ha =
scritto:
>=20
> On 01/25/2017 01:46 AM, Paolo Valente wrote:
>>=20
>>> Il giorno 23 gen 2017, alle ore 18:42, Jens Axboe <axboe@fb.com> ha =
scritto:
>>>=20
>>> On 01/23/2017 10:04 AM, Paolo Valente wrote:
>>>>=20
>>>>> Il giorno 18 gen 2017, alle ore 17:21, Jens Axboe <axboe@fb.com> =
ha scritto:
>>>>>=20
>>>>> 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).
>>>>>=20
>>>>> 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.
>>>>>=20
>>>>=20
>>>> 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?
>>>=20
>>> 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.
>>>=20
>>=20
>> 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?
>>=20
>> 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?
>=20
> 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.
>=20
> --=20
> Jens Axboe