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

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Message-ID: <20250211105713.GD29593@noisy.programming.kicks-ass.net>
Date: Tue, 11 Feb 2025 11:57:13 +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 Tue, Feb 11, 2025 at 09:21:38AM +0100, Sebastian Andrzej Siewior wrote:

> So with LAZY_PREEMPT (not that one that was merged upstream, its
> predecessor) we had a counter similar to the preemption counter. On each
> rt_spin_lock() the counter was incremented, on each rt_spin_unlock() the
> counter was decremented. Once the counter hit zero and the lazy preempt
> flag was set (which was only set on schedule requests by SCHED_OTHER
> tasks), we scheduled.
> This improved the performance as we didn't schedule() while holding a
> spinlock_t and then bump into the same lock in the next task.
> 
> We don't follow this behaviour exactly today.

I think I send some hackery Mike's way to implement that at some point.

IIRC it wasn't an obvious win. Anyway its not too hard to do.