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

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Message-ID: <aAZiwGy1A7J4spDk@kbusch-mbp.dhcp.thefacebook.com>
Date: Mon, 21 Apr 2025 09:22:40 -0600
From: Keith Busch <kbusch@...nel.org>
To: Matt Fleming <mfleming@...udflare.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kernel-team <kernel-team@...udflare.com>
Subject: Re: 10x I/O await times in 6.12

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.