linux-kernel - Re: [PATCH] sched: Optimize __calc

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Message-ID: <YDliCOPy9s1RdLwd@hirez.programming.kicks-ass.net>
Date:   Fri, 26 Feb 2021 22:03:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, Clement Courbet <courbet@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>
Subject: Re: [PATCH] sched: Optimize __calc_delta.

On Fri, Feb 26, 2021 at 11:52:39AM -0800, Josh Don wrote:
> From: Clement Courbet <courbet@...gle.com>
> 
> A significant portion of __calc_delta time is spent in the loop
> shifting a u64 by 32 bits. Use a __builtin_clz instead of iterating.
> 
> This is ~7x faster on benchmarks.

Have you tried on hardware without such fancy instructions?