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

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Message-ID: <2024081236-entourage-matter-37c6@gregkh>
Date: Mon, 12 Aug 2024 14:42:08 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Christian Loehle <christian.loehle@....com>
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 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?

thanks,

greg k-h