linux-kernel - Re: [PATCH 6.1.y] cpuidle: teo: Remove recent intercepts metric

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Message-ID: <86d4bf8f-186d-4a65-9f06-3e4d5a2a2e1c@arm.com>
Date: Tue, 13 Aug 2024 14:18:53 +0100
From: Christian Loehle <christian.loehle@....com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, stable@...r.kernel.org,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 vincent.guittot@...aro.org, qyousef@...alina.io, peterz@...radead.org,
 daniel.lezcano@...aro.org, ulf.hansson@...aro.org, anna-maria@...utronix.de,
 dsmythies@...us.net, kajetan.puchalski@....com, lukasz.luba@....com,
 dietmar.eggemann@....com
Subject: Re: [PATCH 6.1.y] cpuidle: teo: Remove recent intercepts metric

On 8/12/24 13:42, Greg KH wrote:
> On Mon, Aug 05, 2024 at 03:58:09PM +0100, Christian Loehle wrote:
>> commit 449914398083148f93d070a8aace04f9ec296ce3 upstream.
>>
>> The logic for recent intercepts didn't work, there is an underflow
>> of the 'recent' value that can be observed during boot already, which
>> teo usually doesn't recover from, making the entire logic pointless.
>> Furthermore the recent intercepts also were never reset, thus not
>> actually being very 'recent'.
>>
>> Having underflowed 'recent' values lead to teo always acting as if
>> we were in a scenario were expected sleep length based on timers is
>> too high and it therefore unnecessarily selecting shallower states.
>>
>> Experiments show that the remaining 'intercept' logic is enough to
>> quickly react to scenarios in which teo cannot rely on the timer
>> expected sleep length.
>>
>> See also here:
>> https://lore.kernel.org/lkml/0ce2d536-1125-4df8-9a5b-0d5e389cd8af@arm.com/
>>
>> Fixes: 77577558f25d ("cpuidle: teo: Rework most recent idle duration values treatment")
>> Link: https://patch.msgid.link/20240628095955.34096-3-christian.loehle@arm.com
>> Signed-off-by: Christian Loehle <christian.loehle@....com>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> ---
>>  drivers/cpuidle/governors/teo.c | 79 ++++++---------------------------
>>  1 file changed, 14 insertions(+), 65 deletions(-)
> 
> We can't just take a 6.1.y backport without newer kernels also having
> this fix.  Can you resend this as backports for all relevant kernels
> please?

Hi Greg,
the email thread might've looked a bit strange to you but as I wrote
in a previous reply:
https://lore.kernel.org/linux-pm/20240628095955.34096-1-christian.loehle@arm.com/T/#ma5bcd00c4b0ffa1fc34e8d7fa237b8de4ee8a25c
@stable
4b20b07ce72f cpuidle: teo: Don't count non-existent intercepts
449914398083 cpuidle: teo: Remove recent intercepts metric
0a2998fa48f0 Revert: "cpuidle: teo: Introduce util-awareness"
apply as-is to
linux-6.10.y
linux-6.6.y
for linux-6.1.y only 449914398083 ("cpuidle: teo: Remove recent intercepts metric")
is relevant, I'll reply with a backport.

Ideally I would wait for an Ack from Rafael though.
Hopefully that makes more sense then.