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

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Message-ID: <4dc89f6c-ab93-d3e7-5b5a-4b2f34e2fcac@kernel.dk>
Date:   Fri, 14 Jul 2023 08:43:48 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Chengming Zhou <chengming.zhou@...ux.dev>,
        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 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.

-- 
Jens Axboe