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

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Date:   Tue, 31 Jul 2018 14:16:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     axboe@...nel.dk, bart.vanassche@....com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] blk-mq: clean up the hctx restart

On Tue, Jul 31, 2018 at 01:19:42PM +0800, jianchao.wang wrote:
> Hi Ming
> 
> On 07/31/2018 12:58 PM, Ming Lei wrote:
> > On Tue, Jul 31, 2018 at 12:02:15PM +0800, Jianchao Wang wrote:
> >> Currently, we will always set SCHED_RESTART whenever there are
> >> requests in hctx->dispatch, then when request is completed and
> >> freed the hctx queues will be restarted to avoid IO hang. This
> >> is unnecessary most of time. Especially when there are lots of
> >> LUNs attached to one host, the RR restart loop could be very
> >> expensive.
> > 
> > The big RR restart loop has been killed in the following commit:
> > 
> > commit 97889f9ac24f8d2fc8e703ea7f80c162bab10d4d
> > Author: Ming Lei <ming.lei@...hat.com>
> > Date:   Mon Jun 25 19:31:48 2018 +0800
> > 
> >     blk-mq: remove synchronize_rcu() from blk_mq_del_queue_tag_set()
> > 
> > 
> 
> Oh, sorry, I didn't look into this patch due to its title when iterated the mail list,
> therefore I didn't realize the RR restart loop has already been killed. :)
> 
> The RR restart loop could ensure the fairness of sharing some LLDD resource,
> not just avoid IO hung. Is it OK to kill it totally ?

Yeah, it is, also the fairness might be improved a bit by the way in
commit 97889f9ac24f8d2fc, especially inside driver tag allocation
algorithem.

Thanks,
Ming