linux-kernel - Re: [PATCHSET v6 0/4] Split iowait into two states

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Message-ID: <8380f7f3-fd9f-45a0-b66b-85ec0b5d0144@gmail.com>
Date: Mon, 31 Mar 2025 10:02:51 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, tglx@...utronix.de,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCHSET v6 0/4] Split iowait into two states

On 8/19/24 16:39, Jens Axboe wrote:
> Hi,
> 
> This is v6 of the patchset where the current in_iowait state is split
> into two parts:
> 
> 1) The "task is sleeping waiting on IO", and would like cpufreq goodness
>     in terms of sleep and wakeup latencies.
> 2) The above, and also accounted as such in the iowait stats.
> 
> The current ->in_iowait covers both, this series splits it into two types
> of state so that each can be controlled seperately.
> 
> Patches 1..3 are prep patches, changing the type of
> task_struct->nr_iowait and adding helpers to manipulate the iowait counts.
> 
> Patch 4 does the actual splitting.
> 
> This has been sitting for a while, would be nice to get this queued up
> for 6.12. Comments welcome!

Good day,

Did anything good happened with these patches or related work?
Christian?

Reminder: the goal is to let io_uring to keep using iowait boosting
but avoid reporting it in the iowait stats, because the jump in the
stat spooks users. I know at least several users carrying out of tree
patches to work it around. And, apparently, disabling the boosting
causes perf regressions.

I'm reading through the thread, but unless I missed something, it looks
like the patchset is actually aligned with future plans on iowait
mentioned in the thread, in a sense that it reduces the exposure to
the user space, and, when it's time, a better approach will be able
replaces it with no visible effect to the user.

On the other hand, there seems to be a work around io_uring patch
queued for, which I quite dislike from io_uring perspective but also
because it exposes even more of iowait to the user.
I can understand why it's there, it has been over a year since v1,
but maybe we can figure something out before it's released? Would
it be fine to have something similar to this series? Any other
ideas?

-- 
Pavel Begunkov