linux-kernel - Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

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Message-ID: <20231026105944.GJ33965@noisy.programming.kicks-ass.net>
Date:   Thu, 26 Oct 2023 12:59:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-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, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.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@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Youssef Esmat <youssefesmat@...omium.org>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

On Wed, Oct 25, 2023 at 11:54:13PM -0400, Steven Rostedt wrote:

>  static void extend(void)
>  {
> 	rseq_map.cr_flags = 1;
>  }
> 
>  static void unextend(void)
>  {
> 	unsigned long prev;
> 
> 	prev = xchg(&rseq_map.cr_flags, 0);

So you complain about overhead and then you add one of the most
expensive ops possible here? xchg has an implicit LOCK prefix and you
really don't need LOCK prefix here.

> 	if (prev & 2) {
> 		tracefs_printf(NULL, "Yield!\n");
> 		sched_yield();
> 	}
>  }