linux-kernel - Re: [RFC] blk-mq: clean up the hctx restart

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Date:   Thu, 2 Aug 2018 15:52:00 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [RFC] blk-mq: clean up the hctx restart

On Wed, 2018-08-01 at 16:58 +0800, Ming Lei wrote:
> On Wed, Aug 01, 2018 at 10:17:30AM +0800, jianchao.wang wrote:
> > However, due to the limits in hctx_may_queue, q_b still cannot get the
> > tags. The RR restart also will not wake up q_a.
> > This is unfair for q_a.
> > 
> > When we remove RR restart fashion, at least, the q_a will be waked up by
> > the hctx restart.
> > Is this the improvement of fairness you said in driver tag allocation ?
> 
> I mean the fairness is totally covered by the general tag allocation
> algorithm now, which is sort of FIFO style because of waitqueue, but RR
> restart wakes up queue in the order of request queue.

>From sbitmap.h:

#define SBQ_WAIT_QUEUES 8

What do you think is the effect of your patch if more than eight LUNs are
active and the SCSI queue is full?

Thanks,

Bart.