linux-kernel - Re: [PATCH 00/24] Complete EEVDF

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Message-ID: <345dfc04-753b-4411-be2d-e4f604c1c4b1@arm.com>
Date: Wed, 6 Nov 2024 12:06:44 +0000
From: Luis Machado <luis.machado@....com>
To: Peter Zijlstra <peterz@...radead.org>,
 K Prateek Nayak <kprateek.nayak@....com>
Cc: Saravana Kannan <saravanak@...gle.com>, Samuel Wu <wusamuel@...gle.com>,
 David Dai <davidai@...gle.com>, mingo@...hat.com, juri.lelli@...hat.com,
 vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com,
 linux-kernel@...r.kernel.org, wuyun.abel@...edance.com,
 youssefesmat@...omium.org, tglx@...utronix.de, efault@....de,
 Android Kernel Team <kernel-team@...roid.com>,
 Qais Yousef <qyousef@...gle.com>, Vincent Palomares <paillon@...gle.com>,
 John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH 00/24] Complete EEVDF

Hi,

On 11/6/24 11:09, Peter Zijlstra wrote:
> On Wed, Nov 06, 2024 at 11:49:00AM +0530, K Prateek Nayak wrote:
> 
>> Since delayed entities are still on the runqueue, they can affect PELT
>> calculation. Vincent and Dietmar have both noted this and Peter posted
>> https://lore.kernel.org/lkml/172595576232.2215.18027704125134691219.tip-bot2@tip-bot2/
>> in response but it was pulled out since Luis reported observing -ve
>> values for h_nr_delayed on his setup. A lot has been fixed around
>> delayed dequeue since and I wonder if now would be the right time to
>> re-attempt h_nr_delayed tracking.
> 
> Yeah, it's something I meant to get back to. I think the patch as posted
> was actually right and it didn't work for Luis because of some other,
> since fixed issue.
> 
> But I might be misremembering things. I'll get to it eventually :/

Sorry for the late reply, I got sidetracked on something else.

There have been a few power regressions (based on our Pixel6-based testing) due
to the delayed-dequeue series.

The main one drove the frequencies up due to an imbalance in the uclamp inc/dec
handling. That has since been fixed by "[PATCH 10/24] sched/uclamg: Handle delayed dequeue". [1]

The bug also made it so disabling DELAY_DEQUEUE at runtime didn't fix things, because the
imbalance/stale state would be perpetuated. Disabling DELAY_DEQUEUE before boot did fix things.

So power use was brought down by the above fix, but some issues still remained, like the
accounting issues with h_nr_running and not taking sched_delayed tasks into account.

Dietmar addressed some of it with "kernel/sched: Fix util_est accounting for DELAY_DEQUEUE". [2]

Peter sent another patch to add accounting for sched_delayed tasks [3]. Though the patch was
mostly correct, under some circumstances [4] we spotted imbalances in the sched_delayed
accounting that slowly drove frequencies up again.

If I recall correctly, Peter has pulled that particular patch from the tree, but we should
definitely revisit it with a proper fix for the imbalance. Suggestion in [5].

[1] https://lore.kernel.org/lkml/20240727105029.315205425@infradead.org/
[2] https://lore.kernel.org/lkml/c49ef5fe-a909-43f1-b02f-a765ab9cedbf@arm.com/
[3] https://lore.kernel.org/lkml/172595576232.2215.18027704125134691219.tip-bot2@tip-bot2/
[4] https://lore.kernel.org/lkml/6df12fde-1e0d-445f-8f8a-736d11f9ee41@arm.com/
[5] https://lore.kernel.org/lkml/6df12fde-1e0d-445f-8f8a-736d11f9ee41@arm.com/