linux-kernel - Re: [RFC][PATCH 2/3] time/tick: Introduce a dyn

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Message-ID: <20250128090737.GX7145@noisy.programming.kicks-ass.net>
Date: Tue, 28 Jan 2025 10:07:37 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Stephen Boyd <sboyd@...nel.org>, Yury Norov <yury.norov@...il.com>,
	Bitao Hu <yaoma@...ux.alibaba.com>,
	Andrew Morton <akpm@...ux-foundation.org>, kernel-team@...roid.com
Subject: Re: [RFC][PATCH 2/3] time/tick: Introduce a dyn_hz boot option

On Mon, Jan 27, 2025 at 10:32:54PM -0800, John Stultz wrote:
> Introduce dyn_hz= option, which allows the actual timer tick to
> be scaled down at boot time.
> 
> This allows kerenls to be built with HZ=1000 but systems to
> effectively run as if HZ=100 if specified.
> 
> The system will still run with the configured HZ value, but
> ticks will just arrive "late".
> 
> The valid values are between 100 and the build time CONFIG_HZ
> value.
> 

> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---
>  include/linux/tick.h      | 11 +++++++++--
>  kernel/Kconfig.hz         |  9 +++++++++
>  kernel/time/tick-common.c | 30 ++++++++++++++++++++++++++++++
>  kernel/time/tick-sched.c  |  4 ++--
>  4 files changed, 50 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index b8ddc8e631a3c..734ee1e08c9ef 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -14,6 +14,13 @@
>  #include <linux/rcupdate.h>
>  #include <linux/static_key.h>
>  
> +#ifdef CONFIG_DYN_HZ
> +extern long long dyn_tick_nsec;
> +#define DYN_TICK_NSEC (dyn_tick_nsec)
> +#else
> +#define DYN_TICK_NSEC TICK_NSEC
> +#endif

My git-grep TICK_NSEC spies a whole bunch of TICK_NSEC users that seem
sad now.

That is, why don't they all need updating?