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

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Date:   Thu, 22 Dec 2016 10:59:27 +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 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.

Paolo