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Date:   Fri, 23 Dec 2016 11:12:41 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Jens Axboe <axboe@...com>, 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 22 dic 2016, alle ore 10:59, Paolo Valente <paolo.valente@...aro.org> ha scritto:
> 
>> 
>> 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.
> 

Jens,
I forgot to add that I'm willing (and would be happy) to propose a fix
to this, and possibly the other problems too, on my own.  Just, I'm
not yet expert enough to do it with having first received some
feedback or instructions from you.  In this specific case, I don't
even know yet whether this is really a bug.

Thanks, and merry Christmas if we don't get in touch before,
Paolo

> Paolo
> 
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