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=-6.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 92184C43457 for ; Wed, 14 Oct 2020 08:33:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 147DE20BED for ; Wed, 14 Oct 2020 08:33:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="uNsVFpZg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728736AbgJNIda (ORCPT ); Wed, 14 Oct 2020 04:33:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60858 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728435AbgJNId3 (ORCPT ); Wed, 14 Oct 2020 04:33:29 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A6CD4C051111 for ; Wed, 14 Oct 2020 01:33:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=SZk4ENvjqBcp2NaJaFEexFN2q87zoKwvZr4D/rU957Y=; b=uNsVFpZgwBoK4ErFiymIcoEXxz RptICUcU6wtLHNLS/oS77zev1GeDgu34UOaCUA65/TzwinCwQ2MRlDiH3W3wMCkT8n65S0iTL627f kU9A4+fpfYqUgXoIyNFZHEbIZwuw+nSHoNxp9BZ53J1lxUtLSvEZeMUtF+wCM0alN2OxDi7ORDrq8 CwobtqODxu/YBm3zTR0tM8ynA3EvJNZxIewQZitvFRgcLC48j8S7z40c9trB5aUgFcQR2vPxVh0zF r7a/NIeMJqh2nN/KhZDT5MicqxgVHs7QXQVRknyaYD4NqEPQznKuLT3S04Mmszb6krfpgjyEBQUXa K2WatgHQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kScDt-0006Fw-Hs; Wed, 14 Oct 2020 08:33:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 20C5F306BCA; Wed, 14 Oct 2020 10:33:21 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 13E47203CD921; Wed, 14 Oct 2020 10:33:21 +0200 (CEST) Date: Wed, 14 Oct 2020 10:33:21 +0200 From: Peter Zijlstra To: Marcelo Tosatti Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Nitesh Narayan Lal , Peter Xu Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task Message-ID: <20201014083321.GA2628@hirez.programming.kicks-ass.net> References: <20201007180151.623061463@redhat.com> <20201007180229.724302019@redhat.com> <20201008122256.GW2628@hirez.programming.kicks-ass.net> <20201008175409.GB14207@fuller.cnet> <20201008195444.GB86389@lothringen> <20201013171328.GA19284@fuller.cnet> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201013171328.GA19284@fuller.cnet> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 13, 2020 at 02:13:28PM -0300, Marcelo Tosatti wrote: > > Yes but if the task isn't running, run_posix_cpu_timers() doesn't have > > anything to elapse. So indeed we can spare the IPI if the task is not > > running. Provided ordering makes sure that the task sees the new dependency > > when it schedules in of course. > > True. > > * p->on_cpu <- { 0, 1 }: > * > * is set by prepare_task() and cleared by finish_task() such that it will be > * set before p is scheduled-in and cleared after p is scheduled-out, both > * under rq->lock. Non-zero indicates the task is running on its CPU. > > > CPU-0 (tick_set_dep) CPU-1 (task switch) > > STORE p->tick_dep_mask > smp_mb() (atomic_fetch_or()) > LOAD p->on_cpu > > > context_switch(prev, next) > STORE next->on_cpu = 1 > ... [*] > > LOAD current->tick_dep_mask > That load is in tick_nohz_task_switch() right? (which BTW is placed completely wrong) You could easily do something like the below I suppose. diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 81632cd5e3b7..2a5fafe66bb0 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -410,6 +410,14 @@ void __tick_nohz_task_switch(void) ts = this_cpu_ptr(&tick_cpu_sched); if (ts->tick_stopped) { + /* + * tick_set_dep() (this) + * + * STORE p->tick_dep_mask STORE p->on_cpu + * smp_mb() smp_mb() + * LOAD p->on_cpu LOAD p->tick_dep_mask + */ + smp_mb(); if (atomic_read(¤t->tick_dep_mask) || atomic_read(¤t->signal->tick_dep_mask)) tick_nohz_full_kick(); re tick_nohz_task_switch() being placed wrong, it should probably be placed before finish_lock_switch(). Something like so. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index cf044580683c..5c92c959824f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4084,6 +4084,7 @@ static struct rq *finish_task_switch(struct task_struct *prev) vtime_task_switch(prev); perf_event_task_sched_in(prev, current); finish_task(prev); + tick_nohz_task_switch(); finish_lock_switch(rq); finish_arch_post_lock_switch(); kcov_finish_switch(current); @@ -4121,7 +4122,6 @@ static struct rq *finish_task_switch(struct task_struct *prev) put_task_struct_rcu_user(prev); } - tick_nohz_task_switch(); return rq; } diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 81632cd5e3b7..c8bddc9fb792 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -402,10 +402,8 @@ void __tick_nohz_task_switch(void) unsigned long flags; struct tick_sched *ts; - local_irq_save(flags); - if (!tick_nohz_full_cpu(smp_processor_id())) - goto out; + return; ts = this_cpu_ptr(&tick_cpu_sched); @@ -414,8 +412,6 @@ void __tick_nohz_task_switch(void) atomic_read(¤t->signal->tick_dep_mask)) tick_nohz_full_kick(); } -out: - local_irq_restore(flags); } /* Get the boot-time nohz CPU list from the kernel parameters. */