linux-kernel - Re: [PATCH 4/4] sched/core: split iowait state into two states

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Date: Wed, 24 Apr 2024 11:08:42 +0100
From: Christian Loehle <christian.loehle@....com>
To: Peter Zijlstra <peterz@...radead.org>, Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
 "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
 daniel.lezcano@...aro.org
Subject: Re: [PATCH 4/4] sched/core: split iowait state into two states

On 24/04/2024 11:01, Peter Zijlstra wrote:
> On Tue, Apr 16, 2024 at 06:11:21AM -0600, Jens Axboe wrote:
>> iowait is a bogus metric, but it's helpful in the sense that it allows
>> short waits to not enter sleep states that have a higher exit latency
>> than would've otherwise have been picked for iowait'ing tasks. However,
>> it's harmless in that lots of applications and monitoring assumes that
>> iowait is busy time, or otherwise use it as a health metric.
>> Particularly for async IO it's entirely nonsensical.
> 
> Let me get this straight, all of this is about working around
> cpuidle menu governor insaity?
> 
> Rafael, how far along are we with fully deprecating that thing? Yes it
> still exists, but should people really be using it still?
> 

Well there is also the iowait boost handling in schedutil and intel_pstate
which, at least in synthetic benchmarks, does have an effect [1].
io_uring (the only user of iowait but not iowait_acct) works around both.

See commit ("8a796565cec3 io_uring: Use io_schedule* in cqring wait")

[1]
https://lore.kernel.org/lkml/20240304201625.100619-1-christian.loehle@arm.com/#t

Kind Regards,
Christian