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

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Message-ID: <2024081410-camping-letter-1d17@gregkh>
Date: Wed, 14 Aug 2024 13:40:34 +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 Tue, Aug 13, 2024 at 02:18:53PM +0100, Christian Loehle wrote:
> 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.

Please send all of these as a patch series for the relevent branches so
we know exactly what is going on...

thanks,

greg k-h