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

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Message-ID: <20180803003507.GE16047@ming.t460p>
Date:   Fri, 3 Aug 2018 08:35:08 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [RFC] blk-mq: clean up the hctx restart

On Thu, Aug 02, 2018 at 05:24:15PM +0000, Bart Van Assche wrote:
> On Fri, 2018-08-03 at 01:17 +0800, Ming Lei wrote:
> > Not mentioning big CPU utilization is consumed unnecessarily for iterating
> > over all queues even though there is only one active queue, is this fair from
> > system view?
> 
> I hope that someone will come up some day with a better solution than the
> list_for_each_entry_rcu_rr() loop. But as long as such a solution has not yet
> been found that loop should stay in order to preserve fair handling of large
> numbers of LUNs.

Again list_for_each_entry_rcu_rr() isn't fair for small number of active
queue in this case, because this way decreases IOPS a lot and consumes CPU
much just for checking all thousands of queues unnecessarily, even though 99%
of them are idle.

Thanks,
Ming