linux-kernel - Re: [RFC PATCH 0/3] blk-mq: rework queue freeze and preempt-only

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Date:   Thu, 20 Sep 2018 19:13:46 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>, axboe@...nel.dk,
        tj@...nel.org, kent.overstreet@...il.com, ming.lei@...hat.com,
        bart.vanassche@....com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] blk-mq: rework queue freeze and preempt-only

On 9/20/18 6:51 PM, jianchao.wang wrote:
> Hi Bart
> 
> On 09/21/2018 01:06 AM, Bart Van Assche wrote:
>> On Thu, 2018-09-20 at 18:18 +0800, Jianchao Wang wrote:
>>> The current queue freeze depending on percpu_ref_kil/reinit has a limit that
>>> we have drain the requests before unfreeze the queue.
>>>
>>> Let's rework the queue freeze feature as following:
>>> 1. introduce __percpu_ref_get_many.
>>>     It is same with original percpu_ref_get_many, but just need callers to provide
>>>     sched rcu critical section. We will put the __percpu_ref_get_many and our own
>>>     condition checking under rcu_read_lock_sched. With this new helper interface,
>>>     we could save an extra rcu_read_lock_sched.
>>> 2. rework the blk_queue_enter as:
>>>     
>>>     rcu_read_lock_sched()
>>>     if condition check true
>>>       __percpu_ref_get_many(&q->q_usage_counter, 1)
>>>     else
>>>       goto wait
>>>     rcu_read_unlock_sched()
>>> 3. use percpu_ref_switch_to_atomic/percpu to switch mode directly.
>>>
>>> Then we could unfreeze the queue w/o draining requests.
>>> In addition, preempt-only mode code could be simplified.
>> Hello Jianchao,
>>                                                                             =
>> Thanks for having taken a look. However, the approach of this patch series
>> may be more complicated than necessary. Had you considered something like
>> the patch below?
> 
> This patch is just a trying that let our life easier w/o changing anything in percpu-ref code. :)
> In fact, the 1st patch which introduces a new non-functional-changing helper interface in
> percpu-ref code is not necessary.
> 
> This patchset just implement our own condition checking in blk_queue_enter instead of depending on
> __PERCPU_REF_DEAD checking in percpu_ref_tryget_live.
> 
> Then we could do more thing here, like:
>   - unfreeze the queue without draining requests.
>   - check whether q->q_usage_counter is zero
>   - add other gate conditions into blk_queue_enter, such as preempt-only, even any others.
> 
> So why not try it ?

Hello Jianchao,

Some time ago Tejun wrote that he wants to keep the dependency of the 
percpu_ref implementation on rcu-sched inside the percpu_ref 
implementation because he would like to have the freedom to switch to 
regular RCU. Your patch series makes the dependency of percpu_ref 
counters on rcu-sched visible outside the percpu_ref implementation. I 
think that's a disadvantage of your approach. Tejun, please correct me 
if I misunderstood you.

Bart.