linux-kernel - Re: 10x I/O await times in 6.12

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Message-ID: <aAbzW1POQP9z5BWS@kbusch-mbp.dhcp.thefacebook.com>
Date: Mon, 21 Apr 2025 19:39:39 -0600
From: Keith Busch <kbusch@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: Matt Fleming <mfleming@...udflare.com>, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team <kernel-team@...udflare.com>,
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: 10x I/O await times in 6.12

On Tue, Apr 22, 2025 at 09:28:02AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/04/21 23:22, Keith Busch 写道:
> > 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.
> 
> This is wrong, scheduled out will clear cached ktime. You can check
> it easily since there are not much caller to clear ktime.

Huh? cond_resched() calls __schedule() directly via
preempt_schedule_common(), which most certainly does not clear the
plug's time.

The timestamp is only invalidated from schedule() or
rt_mutex_post_schedule(). You can check it ... "easily".