linux-kernel - Re: [PATCH] Kconfig.hz: Change default HZ to 1000

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Message-ID: <CAKfTPtCiTz+-4Umcmt4adn3u6r=rcGoSZgNPfuvacCQ0XerKGg@mail.gmail.com>
Date: Mon, 24 Feb 2025 14:16:21 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Andrea Righi <arighi@...dia.com>, 
	Joel Fernandes <joelagnelf@...dia.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, John Stultz <jstultz@...gle.com>, 
	Saravana Kannan <saravanak@...gle.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Frederic Weisbecker <frederic@...nel.org>, linux-kernel@...r.kernel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Kconfig.hz: Change default HZ to 1000

On Mon, 24 Feb 2025 at 00:21, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 02/23/25 11:00, Ingo Molnar wrote:
> >
> > * Andrea Righi <arighi@...dia.com> wrote:
> >
> > > On Wed, Feb 12, 2025 at 09:50:54AM -0500, Joel Fernandes wrote:
> > > > On Mon, Feb 10, 2025 at 12:19:15AM +0000, Qais Yousef wrote:
> > > ...
> > > > > I believe HZ_250 was the default as a trade-off for battery power
> > > > > devices that might not be happy with frequent TICKS potentially draining
> > > > > the battery unnecessarily. But to my understanding the current state of
> > > >
> > > > Actually, on x86, me and Steve did some debug on Chromebooks and we found
> > > > that HZ_250 actually increased power versus higher HZ. This was because
> > > > cpuidle governor changes C states on the tick, and by making it less
> > > > frequent, the CPU could be in a shallow C state for longer.
> > >
> > > FWIW, I found the same about power consumption when we decided to switch to
> > > CONFIG_HZ=1000 in the Ubuntu kernel:
> > > https://discourse.ubuntu.com/t/enable-low-latency-features-in-the-generic-ubuntu-kernel-for-24-04/42255
>
> Thanks for sharing the data Andrea!

I  don't have power figures to share but I'm aligned with the fact
that keeping HZ=250 (4ms) is not sustainable when it comes to handle
task scheduling on devices with 120 fps (~8ms) constraint.

FWIW
Acked-by : Vincent Guittot <vincent.guittot@...aro.org>

>
> >
> > The "HZ=1000 reduces power consumption or keeps it the same" is
> > actually a pretty good argument to change the default to HZ=1000.
> >
> > These experiments and numbers (if any) should be incorporated in the
> > changelog prominently - as actual data and the Kconfig decisions made
> > by major distros will, most of the time, be superior to meta analysis
> > that seems to be the changelog right now.
>
> I will update the commit message to incorporate data and the feedback received.
>
> Thanks!
>
> --
> Qais Yousef