linux-kernel - Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle

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Message-ID: <1512576435.3297.3.camel@wdc.com>
Date:   Wed, 6 Dec 2017 16:07:17 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "ming.lei@...hat.com" <ming.lei@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "axboe@...com" <axboe@...com>, "hare@...e.com" <hare@...e.com>,
        "holger@...lied-asynchrony.com" <holger@...lied-asynchrony.com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>
Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle

On Wed, 2017-12-06 at 09:52 +0800, Ming Lei wrote:
> On Wed, Dec 06, 2017 at 12:28:25AM +0800, Ming Lei wrote:
> > On Tue, Dec 05, 2017 at 04:08:20PM +0000, Bart Van Assche wrote:
> > > The patch below is not a full solution but resulted in a significant
> > > improvement in my tests:
> > > 
> > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > > index 69e3226e66ca..9d86876ec503 100644
> > > --- a/block/blk-mq-sched.c
> > > +++ b/block/blk-mq-sched.c
> > > @@ -226,6 +226,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> > >  		 * TODO: get more budgets, and dequeue more requests in
> > >  		 * one time.
> > >  		 */
> > > +		blk_mq_sched_mark_restart_hctx(hctx);
> > >  		blk_mq_do_dispatch_ctx(hctx);
> > >  	} else {
> > >  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
> 
> BTW, this kind of change can't cover scsi_set_blocked() which is
> triggered by timeout, scsi dispatch failure. You will see that
> easily if you run the SCSI test script I provided in the commit log.

Hello Ming,

I am aware that the above change does not cover all cases. That's why I wrote
in my previous e-mail that that patch is not a full solution. The reason I
posted that change anyway is because I prefer a solution that is not based on
delayed queue runs over a solution that is based on delayed queue runs
(blk_mq_delay_run_hw_queue()). My concern is that performance of a solution
based on delayed queue runs will be suboptimal.

Bart.