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.
