linux-kernel - Re: [PATCH 6/8] blk-mq-sched: add framework for MQ capable IO schedulers

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Date:   Tue, 17 Jan 2017 10:17:09 +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>, osandov@...com
Subject: Re: [PATCH 6/8] blk-mq-sched: add framework for MQ capable IO schedulers


> Il giorno 17 gen 2017, alle ore 03:47, Jens Axboe <axboe@...com> ha scritto:
> 
> On 12/22/2016 02:59 AM, Paolo Valente wrote:
>> 
>>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@...com> ha scritto:
>>> 
>>> This adds a set of hooks that intercepts the blk-mq path of
>>> allocating/inserting/issuing/completing requests, allowing
>>> us to develop a scheduler within that framework.
>>> 
>>> We reuse the existing elevator scheduler API on the registration
>>> side, but augment that with the scheduler flagging support for
>>> the blk-mq interfce, and with a separate set of ops hooks for MQ
>>> devices.
>>> 
>>> Schedulers can opt in to using shadow requests. Shadow requests
>>> are internal requests that the scheduler uses for for the allocate
>>> and insert part, which are then mapped to a real driver request
>>> at dispatch time. This is needed to separate the device queue depth
>>> from the pool of requests that the scheduler has to work with.
>>> 
>>> Signed-off-by: Jens Axboe <axboe@...com>
>>> 
>> ...
>> 
>>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
>>> new file mode 100644
>>> index 000000000000..b7e1839d4785
>>> --- /dev/null
>>> +++ b/block/blk-mq-sched.c
>> 
>>> ...
>>> +static inline bool
>>> +blk_mq_sched_allow_merge(struct request_queue *q, struct request *rq,
>>> +			 struct bio *bio)
>>> +{
>>> +	struct elevator_queue *e = q->elevator;
>>> +
>>> +	if (e && e->type->ops.mq.allow_merge)
>>> +		return e->type->ops.mq.allow_merge(q, rq, bio);
>>> +
>>> +	return true;
>>> +}
>>> +
>> 
>> Something does not seem to add up here:
>> e->type->ops.mq.allow_merge may be called only in
>> blk_mq_sched_allow_merge, which, in its turn, may be called only in
>> blk_mq_attempt_merge, which, finally, may be called only in
>> blk_mq_merge_queue_io.  Yet the latter may be called only if there is
>> no elevator (line 1399 and 1507 in blk-mq.c).
>> 
>> Therefore, e->type->ops.mq.allow_merge can never be called, both if
>> there is and if there is not an elevator.  Be patient if I'm missing
>> something huge, but I thought it was worth reporting this.
> 
> I went through the current branch, and it seems mostly fine. There was
> a double call to allow_merge() that I killed in the plug path, and one
> set missing in blk_mq_sched_try_merge(). The rest looks OK.
> 

Yes, I missed a path, sorry.  I'm happy that at least your check has
not been a waste of time for other reasons.

Thanks,
Paolo

> -- 
> Jens Axboe
>