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

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Date:   Wed, 1 Aug 2018 10:17:30 +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

Thanks for your kindly response.

On 07/31/2018 02:16 PM, Ming Lei wrote:
> 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.
> 

Would you mind to detail more here ?

Regarding the driver tag case:
For example:

q_a         q_b        q_c       q_d
hctx0       hctx0      hctx0     hctx0

                 tags

Total number of tags is 32
All of these 4 q are active.

So every q has 8 tags.

If all of these 4 q have used up their 8 tags, they have to wait.

When part of the in-flight requests q_a are completed, tags are freed.
but the __sbq_wake_up doesn't wake up the q_a, it may wake up q_b.
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 ?

Think further, it seems that it only works for case with io scheduler.
w/o io scheduler, tasks will wait in blk_mq_get_request. restart hctx will
not work there.

Thanks
Jianchao