linux-kernel - Re: [RFC][PATCH 0/3] DynamicHZ: Configuring the timer tick rate at boot time

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Message-ID: <20250210011413.whnkxo4modrx6gbc@airbuntu>
Date: Mon, 10 Feb 2025 01:14:13 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, John Stultz <jstultz@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, 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 0/3] DynamicHZ: Configuring the timer tick rate at
 boot time

On 02/03/25 12:14, Peter Zijlstra wrote:

> The whole preemption thing could probably be replaced with HRTICK (which
> might be suffering from bitrot), but the problem has always been with
> hrtimers being too expensive (on x86). But ideally we'd move away from
> tick based preemption.

I do think HRTICK must be on by default. Precise time slices are important.
Especially if your plans to attach changing of base_slice to
sched_attr->runtime are still happening. People will find lots of surprises
when they ask for 500us runtime but end up running (much) longer, especially
with the default 4ms TICK value (which I just posted a patch to change it).

Can we re-evaluate the HRTICK impact?