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

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Message-ID: <4f804d53-6f6c-402c-b8db-db6ab8161f11@kernel.dk>
Date: Wed, 4 Sep 2024 08:49:30 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, rafael@...nel.org,
 daniel.lezcano@...aro.org, linux-pm@...r.kernel.org
Subject: Re: [PATCHSET v6 0/4] Split iowait into two states

On 9/4/24 8:41 AM, Jens Axboe wrote:
> On 9/4/24 8:28 AM, Peter Zijlstra wrote:
>> On Mon, Aug 19, 2024 at 09:39:45AM -0600, 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.
>>
>> Yeah, but *WHY* !?!? I have some vague memories from last time around,
>> but patches should really keep this information.
> 
> To decouple the frequency boost on short waits from the accounting side,
> as lots of tooling equates iowait time with busy time and reports it as
> such. Yeah that's garbage and a reporting issue, but decades of
> education hasn't really improved on that. We should've dumped iowait
> once we moved away from 1-2 processor system or had preemptible kernels,
> but alas we did not and here we are in 2024.

Forgot to mention, it's not *just* an educational thing - lots services
of services do mixed network and disk IO, obviously, and they do have
some interest in retaining iowait metrics on the disk side.

-- 
Jens Axboe