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Date: Wed, 24 Apr 2024 11:08:42 +0100
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Subject: Re: [PATCH 4/4] sched/core: split iowait state into two states
To: Peter Zijlstra <peterz@infradead.org>, Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de,
 "Rafael J. Wysocki" <rjw@rjwysocki.net>, linux-pm@vger.kernel.org,
 daniel.lezcano@linaro.org
References: <20240416121526.67022-1-axboe@kernel.dk>
 <20240416121526.67022-5-axboe@kernel.dk>
 <20240424100127.GV40213@noisy.programming.kicks-ass.net>
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From: Christian Loehle <christian.loehle@arm.com>
In-Reply-To: <20240424100127.GV40213@noisy.programming.kicks-ass.net>
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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