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. Thanks.
