linux-kernel - Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

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Message-ID: <20250206142717.GS7145@noisy.programming.kicks-ass.net>
Date: Thu, 6 Feb 2025 15:27:17 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Prakash Sangappa <prakash.sangappa@...cle.com>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ankur Arora <ankur.a.arora@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
	hpa@...or.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
	bharata@....com, raghavendra.kt@....com,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Wilk <konrad.wilk@...cle.com>, jgross@...e.com,
	Andrew.Cooper3@...rix.com, Vineeth Pillai <vineethrp@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Clark Williams <clark.williams@...il.com>, daniel.wagner@...e.com,
	Joseph Salisbury <joseph.salisbury@...cle.com>, broonie@...il.com
Subject: Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

On Thu, Feb 06, 2025 at 03:22:34PM +0100, Sebastian Andrzej Siewior wrote:
>  Then this feature adds 20us on top?

The point has always been for the number to be < the observable
scheduling latency.

I'm not sure what that number is, and it is always hardware dependent. I
measured it on a random test box when I did the prototype a long while
ago, and ended up at 50us, but for all I know that machine was running a
lockdep enabled kernel at the time (won't be the first and certainly
won't be the last time I try and do a performance measurement on a debug
kernel).

That was not the important part -- but everybody fixates on the number,
instead of the intent.

I'm assuming you have a recent number around -- what's sane? 5us, less?