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=-8.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 6CEEAC433E7 for ; Thu, 8 Oct 2020 18:05:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E90E920725 for ; Thu, 8 Oct 2020 18:05:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="En0Rl1jy" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730795AbgJHSF1 (ORCPT ); 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Thu, 8 Oct 2020 18:05:22 +0000 (UTC) Received: from fuller.cnet (ovpn-112-7.gru2.redhat.com [10.97.112.7]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 955B75C1BD; Thu, 8 Oct 2020 18:05:16 +0000 (UTC) Received: by fuller.cnet (Postfix, from userid 1000) id 280094168BA3; Thu, 8 Oct 2020 14:54:09 -0300 (-03) Date: Thu, 8 Oct 2020 14:54:09 -0300 From: Marcelo Tosatti To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Frederic Weisbecker , Nitesh Narayan Lal , Peter Xu Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task Message-ID: <20201008175409.GB14207@fuller.cnet> References: <20201007180151.623061463@redhat.com> <20201007180229.724302019@redhat.com> <20201008122256.GW2628@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201008122256.GW2628@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 08, 2020 at 02:22:56PM +0200, Peter Zijlstra wrote: > On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote: > > When adding a tick dependency to a task, its necessary to > > wakeup the CPU where the task resides to reevaluate tick > > dependencies on that CPU. > > > > However the current code wakes up all nohz_full CPUs, which > > is unnecessary. > > > > Switch to waking up a single CPU, by using ordering of writes > > to task->cpu and task->tick_dep_mask. > > > > From: Frederic Weisbecker > > Suggested-by: Peter Zijlstra > > Signed-off-by: Frederic Weisbecker > > Signed-off-by: Marcelo Tosatti > > > > Index: linux-2.6/kernel/time/tick-sched.c > > =================================================================== > > --- linux-2.6.orig/kernel/time/tick-sched.c > > +++ linux-2.6/kernel/time/tick-sched.c > > @@ -274,6 +274,31 @@ void tick_nohz_full_kick_cpu(int cpu) > > irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu); > > } > > > > +static void tick_nohz_kick_task(struct task_struct *tsk) > > +{ > > + int cpu = task_cpu(tsk); > > + > > + /* > > + * If the task concurrently migrates to another cpu, > > + * we guarantee it sees the new tick dependency upon > > + * schedule. > > + * > > + * > > + * set_task_cpu(p, cpu); > > + * STORE p->cpu = @cpu > > + * __schedule() (switch to task 'p') > > + * LOCK rq->lock > > + * smp_mb__after_spin_lock() STORE p->tick_dep_mask > > + * tick_nohz_task_switch() smp_mb() (atomic_fetch_or()) > > + * LOAD p->tick_dep_mask LOAD p->cpu > > + */ > > + > > + preempt_disable(); > > + if (cpu_online(cpu)) > > + tick_nohz_full_kick_cpu(cpu); > > + preempt_enable(); > > +} > > So we need to kick the CPU unconditionally, or only when the task is > actually running? AFAICT we only care about current->tick_dep_mask. tick is necessary to execute run_posix_cpu_timers, from tick interrupt, even if task is not running.