linux-kernel - Re: [PATCH] sched: add unlikey branch hints to several system calls

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Message-ID: <20250219143704.GF34567@noisy.programming.kicks-ass.net>
Date: Wed, 19 Feb 2025 15:37:04 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Colin Ian King <colin.i.king@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, 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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: add unlikey branch hints to several system calls

On Wed, Feb 19, 2025 at 02:24:23PM +0000, Colin Ian King wrote:
> Adding an unlikely() hint on early error return paths improves the
> run-time performance of several sched related system calls.
> 
> Benchmarking on an i9-12900 shows the following per system call
> performance improvements:
> 
> 		       before     after     improvement
> sched_getattr          182.4ns    170.6ns      ~6.5%
> sched_setattr          284.3ns    267.6ns      ~5.9%
> sched_getparam         161.6ns    148.1ns      ~8.4%
> sched_setparam        1265.4ns   1227.6ns      ~3.0%
> sched_getscheduler     129.4ns    118.2ns      ~8.7%
> sched_setscheduler    1237.3ns   1216.7ns      ~1.7%
> 
> Results are based on running 20 tests with turbo disabled (to reduce
> clock freq turbo changes), with 10 second run per test based on the
> number of system calls per second. The % standard deviation of the
> measurements for the 20 tests was 0.05% to 0.40%, so the results are
> reliable.
> 
> Tested on kernel build with gcc 14.2.1

Nice, thanks!