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

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Message-ID: <5be1cba6-b141-3a05-f801-3af7d2092674@linux.dev>
Date:   Fri, 14 Jul 2023 19:31:46 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, 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 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.

Thanks.