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@xxxxxxxxx/ >>>> >>>> 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!
