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

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Message-ID: <30eeae06-0d8a-4968-ba57-d723162a0782@kernel.dk>
Date: Wed, 4 Sep 2024 08:41:23 -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 8:28 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.

>> 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!
> 
> Ufff, and all this because menu-governor does something insane :-(
> 
> Rafael, why can't we simply remove this from menu? All the nr_iowait*()
> users are basically broken and I would much rather fix broken rather
> than work around broken like this.
> 
> That is, from where I'm sitting this all makes the io-wait situation far
> worse instead of better.

IMHO what we need is a way to propagate expected wait times for a
sleeper. Right now iowait serves this purpose in a very crude way, in
that it doesn't really tell you the expected wait, just that it's a
short one.

If we simply remove iowait frequency boosting, then we'll have big
regressions particularly for low/sync storage IO.

-- 
Jens Axboe