linux-kernel - Re: [PATCH v5] blk-mq: fix start_time_ns and alloc_time

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Message-ID: <8631f3e6-fef1-bff1-a793-b6e7410802b1@linux.dev>
Date:   Fri, 14 Jul 2023 22:49:13 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Cc:     hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, ming.lei@...hat.com,
        zhouchengming@...edance.com
Subject: Re: [PATCH v5] blk-mq: fix start_time_ns and alloc_time_ns for
 pre-allocated rq

On 2023/7/14 22:43, Jens Axboe wrote:
> On 7/14/23 5:31?AM, Chengming Zhou wrote:
>> On 2023/7/14 01:58, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Thu, Jul 13, 2023 at 08:25:50PM +0800, Chengming Zhou wrote:
>>>> Ok, this version will only get time stamp once for one request, it's actually
>>>> not worse than the current code, which will get start time stamp once for each
>>>> request even in the batch allocation.
>>>>
>>>> But yes, maybe we can also set the start time stamp in the batch mode, and only
>>>> update the time stamp in the block case, like you said, has better performance.
>>>>
>>>> The first version [1] I posted actually just did this, in which use a nr_flush counter
>>>> in plug to indicate that we blocked & flushed plug. Tejun and I think it seems fragile.
>>>> So go to this way that only set time stamp once when the request actually used.
>>>>
>>>> [1] https://lore.kernel.org/all/20230601053919.3639954-1-chengming.zhou@linux.dev/
>>>>
>>>> Another way I can think of is to make rq_qos_throttle() return a bool to indicate
>>>> if it blocked. Tejun and Jens, how do you think about this way?
>>>>
>>>> Although it's better performance, in case of preemption, the time stamp maybe not accurate.
>>>
>>> Trying to manually optimized timestamp reads seems like a bit of fool's
>>> errand to me. I don't think anyone cares about nanosec accuracy, so there
>>> are ample opportunities for generically caching timestamp so that we don't
>>> have to contort code to optimzie timestamp calls.
>>>
>>> It's a bit out of scope for this patchset but I think it might make sense to
>>> build a timestamp caching infrastructure. The cached timestamp can be
>>> invalidated on context switches (block layer already hooks into them) and
>>> issue and other path boundaries (e.g. at the end of plug flush).
>>>
>>
>> Yes, this is a really great idea. It has better performance and is
>> more generic.
> 
> Do you want to work on that approach? I pretty much outlined how I think
> it'd work in the previous reply.
> 

Ok, I want to do it. Your outline is clear, will implement and test it.

Thanks!