From: Peter Zijlstra <peterz@infradead.org>
To: Josh Don <joshdon@google.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
Clement Courbet <courbet@google.com>,
Oleg Rombakh <olegrom@google.com>
Subject: Re: [PATCH] sched: Optimize __calc_delta.
Date: Wed, 3 Mar 2021 10:49:27 +0100 [thread overview]
Message-ID: <YD9bp7C9k6ojjq/5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CABk29Nv7iJEcDg3rgSvfTkXEM69ZeLByJAsZYuA5qpdj645nZw@mail.gmail.com>
On Tue, Mar 02, 2021 at 12:55:07PM -0800, Josh Don wrote:
> On Fri, Feb 26, 2021 at 1:03 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Feb 26, 2021 at 11:52:39AM -0800, Josh Don wrote:
> > > From: Clement Courbet <courbet@google.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?
>
> Was not able to find any on hand unfortunately. Clement did rework the
> patch to use fls() instead, and has benchmarks for the generic and asm
> variations. All of which are faster than the loop. In my next reply,
> I'll include the updated patch inline.
Excellent; I have some vague memories where using fls ended up slower
for some ARMs, but I can't seem to remember enough to even Google it :/
next prev parent reply other threads:[~2021-03-03 14:33 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-26 19:52 [PATCH] sched: Optimize __calc_delta Josh Don
2021-02-26 21:03 ` Peter Zijlstra
2021-03-02 20:55 ` Josh Don
2021-03-02 20:57 ` Josh Don
2021-03-03 9:56 ` Peter Zijlstra
2021-03-03 21:59 ` Josh Don
2021-03-03 22:46 ` [PATCH v2] " Josh Don
2021-03-04 8:31 ` Peter Zijlstra
2021-03-04 17:34 ` Nick Desaulniers
2021-03-04 18:24 ` Sedat Dilek
2021-03-04 19:21 ` Sedat Dilek
2021-03-05 1:04 ` Josh Don
2021-03-05 17:13 ` David Laight
2021-03-10 11:26 ` [tip: sched/core] sched: Optimize __calc_delta() tip-bot2 for Clement Courbet
2021-03-03 10:02 ` [PATCH] sched: Optimize __calc_delta Peter Zijlstra
2021-03-03 22:00 ` Josh Don
2021-03-03 9:49 ` Peter Zijlstra [this message]
2021-02-28 15:15 ` [sched] 4112549ee5: WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_fwd_prog_nr[rcutorture] kernel test robot
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