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

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Message-ID: <8a3383e6-2926-6858-d8f2-671f3cb9e460@oracle.com>
Date:   Tue, 31 Jul 2018 13:19:42 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <ming.lei@...hat.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

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 ?

Thanks
Jianchao