linux-kernel - Re: [RFC PATCH 1/2] blk-mq: don't call callbacks for requests that bypassed the scheduler

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Date:   Mon, 30 Aug 2021 09:48:06 +0000
From:   Niklas Cassel <Niklas.Cassel@....com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     Jens Axboe <axboe@...nel.dk>, Bart Van Assche <bvanassche@....org>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Paolo Valente <paolo.valente@...aro.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] blk-mq: don't call callbacks for requests that
 bypassed the scheduler

On Fri, Aug 27, 2021 at 09:28:07PM +0800, Ming Lei wrote:
> On Fri, Aug 27, 2021 at 12:41:31PM +0000, Niklas Cassel wrote:
> > From: Niklas Cassel <niklas.cassel@....com>
> > 
> > Currently, __blk_mq_alloc_request() calls ops.prepare_request and sets
> > RQF_ELVPRIV.
> > 
> > Therefore, (if the request is not a flush) the RQF_ELVPRIV flag will be
> > set for the request in blk_mq_submit_bio(), regardless if the request
> > was submitted to a scheduler, or bypassed the scheduler.
> > 
> > Later, blk_mq_free_request() checks if the RQF_ELVPRIV flag is set,
> > if it is, the ops.finish_request callback will be called.
> > 
> > The problem with this is that the finish_request scheduler callback
> > will be called for requests that bypassed the scheduler.
> > 
> > Fix this by calling the scheduler ops.prepare_request callback, and
> > set the RQF_ELVPRIV flag only immediately before calling the insert
> > callback.
> 
> One request could be inserted more than one times, such as requeue,
> however __blk_mq_alloc_request() is just run once, so is it fine to
> call ->prepare_request more than one time for same request?

Calling ->prepare_request multiple times is fine.
All the different I/O schedulers (BFQ, mq-deadline, kyber)
simply use .prepare_request to clear/set elv->priv to a fixed value.

> 
> Or I am wondering why not call ->prepare_request when the following
> check is true?
> 
> 	if (e && e->type->ops.prepare_request && !op_is_flush(data->cmd_flags) &&
> 		!blk_op_is_passthrough(data->cmd_flags))
> 		e->type->ops.prepare_request()


That might work, and might be a nicer solution indeed.

If a request got plugged, it will be inserted to the scheduler through
blk_flush_plug_list() -> blk_mq_flush_plug_list() -> blk_mq_sched_insert_requests()
which will insert them unconditionally.
In this case. we know that !op_is_flush() (because if it was, blk_mq_submit_bio()
would have inserted directly.)


If we didn't plug, we do blk_mq_sched_insert_request(), which will add it if
blk_mq_sched_bypass_insert() returns false:

blk_mq_sched_bypass_insert() is defined as:

        if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
                return true;
Also in this case. we know that !op_is_flush() (blk_mq_submit_bio() would have
inserted directly.)


So, we could easily add && !blk_op_is_passthrough(data->cmd_flags) to the
->prepare_request condition in blk_mq_rq_ctx_init() like you suggested,
but since the bypass condition also seems to look at RQF_FLUSH_SEQ, wouldn't
we need to add RQF_FLUSH_SEQ to the condition in blk_mq_rq_ctx_init() as well?

This flag is set after blk_mq_rq_ctx_init(). Are we sure that RQF_FLUSH_SEQ
flag will only be set for a request which op_is_flush() returned true?

(If so, then only adding  && !blk_op_is_passthrough(data->cmd_flags) should
be fine.)


Kind regards,
Niklas