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=-16.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 2B843C433FE for ; Fri, 11 Dec 2020 23:14:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00A03230FB for ; Fri, 11 Dec 2020 23:14:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388491AbgLKWWK (ORCPT ); Fri, 11 Dec 2020 17:22:10 -0500 Received: from mail.kernel.org ([198.145.29.99]:50072 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728668AbgLKWVr (ORCPT ); Fri, 11 Dec 2020 17:21:47 -0500 Date: Fri, 11 Dec 2020 23:21:04 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1607725266; bh=QjcAyGeOdHLkJ34+U+ps+ACBJVcdcJxP0G2iwWZMTEg=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=PRfgAHmbCX2a8hpRBXSy39XAT6/7u81neovUyoR2sSu8ilw7JXo66ut5ZhSCdGGns ubI73UWt3LQq5GNf2F5zinRnsgVPvEMxdhOSEMeupcwr8XK9Awy1t/Q/Qd2etWhOWf S9fehJPf1x2x3PJ91qAgnae6+fTmY7u1FEuTeJlfrvzB9Uxi/daVWp+GnHYYkm7k3t FDGPmmaGzMLxAcExtQokLQ+a3q3+0YUWwijXYBx++okR8SwrzhQLGBzhS1MT3hPlTd H7qYHiTzDtkRVLDcR2XnBUNz7FGfYhGXk0EPPzsl5R8YHaDscr57Iv8uOuMfXFymeU rPojYTSKJ+6/Q== From: Frederic Weisbecker To: Thomas Gleixner Cc: LKML , Marco Elver , kasan-dev , Peter Zijlstra , "Paul E. McKenney" , Ingo Molnar , Will Deacon , Naresh Kamboju Subject: Re: [patch 1/3] tick: Remove pointless cpu valid check in hotplug code Message-ID: <20201211222104.GB595642@lothringen> References: <20201206211253.919834182@linutronix.de> <20201206212002.582579516@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201206212002.582579516@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Dec 06, 2020 at 10:12:54PM +0100, Thomas Gleixner wrote: > tick_handover_do_timer() which is invoked when a CPU is unplugged has a > check for cpumask_first(cpu_online_mask) when it tries to hand over the > tick update duty. > > Checking the result of cpumask_first() there is pointless because if the > online mask is empty at this point, then this would be the last CPU in the > system going offline, which is impossible. There is always at least one CPU > remaining. If online mask would be really empty then the timer duty would > be the least of the resulting problems. > > Remove the well meant check simply because it is pointless and confusing. > > Signed-off-by: Thomas Gleixner > --- > kernel/time/tick-common.c | 10 +++------- > 1 file changed, 3 insertions(+), 7 deletions(-) > > --- a/kernel/time/tick-common.c > +++ b/kernel/time/tick-common.c > @@ -407,17 +407,13 @@ EXPORT_SYMBOL_GPL(tick_broadcast_oneshot > /* > * Transfer the do_timer job away from a dying cpu. > * > - * Called with interrupts disabled. Not locking required. If > + * Called with interrupts disabled. No locking required. If > * tick_do_timer_cpu is owned by this cpu, nothing can change it. > */ > void tick_handover_do_timer(void) > { > - if (tick_do_timer_cpu == smp_processor_id()) { > - int cpu = cpumask_first(cpu_online_mask); > - > - tick_do_timer_cpu = (cpu < nr_cpu_ids) ? cpu : > - TICK_DO_TIMER_NONE; > - } > + if (tick_do_timer_cpu == smp_processor_id()) > + tick_do_timer_cpu = cpumask_first(cpu_online_mask); I was about to whine that this randomly chosen CPU may be idle and leave the timekeeping stale until I realized that stop_machine() is running at that time. Might be worth adding a comment about that. Also why not just setting it to TICK_DO_TIMER_NONE and be done with it? Perhaps to avoid that all the CPUs to compete and contend on jiffies update after stop machine? If so: Reviewed-by: Frederic Weisbecker Thanks.