From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 196CBC432C0 for ; Wed, 27 Nov 2019 15:15:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E0B25206F0 for ; Wed, 27 Nov 2019 15:15:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727423AbfK0PPA (ORCPT ); Wed, 27 Nov 2019 10:15:00 -0500 Received: from mail.kernel.org ([198.145.29.99]:44296 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726655AbfK0PPA (ORCPT ); Wed, 27 Nov 2019 10:15:00 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id F1A8B20674; Wed, 27 Nov 2019 15:14:58 +0000 (UTC) Date: Wed, 27 Nov 2019 10:14:56 -0500 From: Steven Rostedt To: Zhenzhong Duan Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de Subject: Re: [PATCH] sched/clock: use static_branch_likely() check at sched_clock_running Message-ID: <20191127101456.2c814108@gandalf.local.home> In-Reply-To: <1574843848-26825-1-git-send-email-zhenzhong.duan@oracle.com> References: <1574843848-26825-1-git-send-email-zhenzhong.duan@oracle.com> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 27 Nov 2019 16:37:28 +0800 Zhenzhong Duan wrote: > sched_clock_running is enabled early at bootup stage and never > disabled. So hints that to compiler by using static_branch_likely() > rather than static_branch_unlikely(). Looks like the confusion was the moving of the "!": - if (unlikely(!sched_clock_running)) + if (!static_branch_unlikely(&sched_clock_running)) Where, it was unlikely that !sched_clock_running would be true, but because the "!" was moved outside the "unlikely()" it makes the test "likely()". That is, if we added an intermediate step, it would have been: if (!likely(sched_clock_running)) which would have prevented the mistake that this patch fixes. Reviewed-by: Steven Rostedt (VMware) -- Steve > > Fixes: 46457ea464f5 ("sched/clock: Use static key for sched_clock_running") > Signed-off-by: Zhenzhong Duan > --- > kernel/sched/clock.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c > index 1152259..12bca64 100644 > --- a/kernel/sched/clock.c > +++ b/kernel/sched/clock.c > @@ -370,7 +370,7 @@ u64 sched_clock_cpu(int cpu) > if (sched_clock_stable()) > return sched_clock() + __sched_clock_offset; > > - if (!static_branch_unlikely(&sched_clock_running)) > + if (!static_branch_likely(&sched_clock_running)) > return sched_clock(); > > preempt_disable_notrace(); > @@ -393,7 +393,7 @@ void sched_clock_tick(void) > if (sched_clock_stable()) > return; > > - if (!static_branch_unlikely(&sched_clock_running)) > + if (!static_branch_likely(&sched_clock_running)) > return; > > lockdep_assert_irqs_disabled(); > @@ -460,7 +460,7 @@ void __init sched_clock_init(void) > > u64 sched_clock_cpu(int cpu) > { > - if (!static_branch_unlikely(&sched_clock_running)) > + if (!static_branch_likely(&sched_clock_running)) > return 0; > > return sched_clock();