On Mon, 21 Apr 2025 at 16:22, Keith Busch <kbusch@xxxxxxxxxx> wrote: > > On Mon, Apr 21, 2025 at 09:53:10AM +0100, Matt Fleming wrote: > > Hey there, > > > > We're moving to 6.12 at Cloudflare and noticed that write await times > > in iostat are 10x what they were in 6.6. After a bit of bpftracing > > (script to find all plug times above 10ms below), it seems like this > > is an accounting error caused by the plug->cur_ktime optimisation > > rather than anything more material. > > > > It appears as though a task can enter __submit_bio() with ->plug set > > and a very stale cur_ktime value on the order of milliseconds. Is this > > expected behaviour? It looks like it leads to inaccurate I/O times. > > There are places with a block plug that call cond_resched(), which > doesn't invalidate the plug's cached ktime. You could end up with a > stale ktime if your process is scheduled out. Is that intentional? I know the cached time is invalidated when calling schedule(). Does the invalidation need to get pushed down into __schedule_loop()?
