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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 0A7B7C433E0 for ; Tue, 7 Jul 2020 09:20:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E33EF2064B for ; Tue, 7 Jul 2020 09:20:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727777AbgGGJUK (ORCPT ); Tue, 7 Jul 2020 05:20:10 -0400 Received: from foss.arm.com ([217.140.110.172]:33460 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725874AbgGGJUK (ORCPT ); Tue, 7 Jul 2020 05:20:10 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 60775C0A; Tue, 7 Jul 2020 02:20:09 -0700 (PDT) Received: from e107158-lin.cambridge.arm.com (e107158-lin.cambridge.arm.com [10.1.195.21]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 45C8E3F68F; Tue, 7 Jul 2020 02:20:08 -0700 (PDT) Date: Tue, 7 Jul 2020 10:20:05 +0100 From: Qais Yousef To: Peter Zijlstra Cc: Dave Jones , Mel Gorman , Linux Kernel , mingo@kernel.org, Linus Torvalds , paul.gortmaker@windriver.com, valentin.schneider@arm.com Subject: Re: weird loadavg on idle machine post 5.7 Message-ID: <20200707092004.jviycpvzb4l5ytiz@e107158-lin.cambridge.arm.com> References: <20200702171548.GA11813@codemonkey.org.uk> <20200702213627.GF3183@techsingularity.net> <20200703090226.GV4800@hirez.programming.kicks-ass.net> <20200703104033.GK117543@hirez.programming.kicks-ass.net> <20200703205153.GA19901@codemonkey.org.uk> <20200706145952.GB597537@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200706145952.GB597537@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20171215 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/06/20 16:59, Peter Zijlstra wrote: [...] > @@ -4104,12 +4108,19 @@ static void __sched notrace __schedule(bool preempt) > local_irq_disable(); > rcu_note_context_switch(preempt); > > + prev_state = prev->state; > + > /* > - * Make sure that signal_pending_state()->signal_pending() below > - * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE) > - * done by the caller to avoid the race with signal_wake_up(). > + * __set_current_state(@state) > + * schedule() signal_wake_up() > + * prev_state = p->state set_tsk_thread_flag(p, TIF_SIGPENDING) > + * wake_up_state() > + * LOCK rq->lock LOCK p->pi_state > + * smp_mb__after_spinlock() smp_mb__after_spinlock() > + * if (signal_pending_state() if (p->state & @state) > + * > * > - * The membarrier system call requires a full memory barrier > + * Also, the membarrier system call requires a full memory barrier > * after coming from user-space, before storing to rq->curr. > */ > rq_lock(rq, &rf); > @@ -4120,10 +4131,30 @@ static void __sched notrace __schedule(bool preempt) > update_rq_clock(rq); > > switch_count = &prev->nivcsw; > - if (!preempt && prev->state) { > - if (signal_pending_state(prev->state, prev)) { > + /* > + * We must re-load p->state in case ttwu_runnable() changed it > + * before we acquired rq->lock. > + */ > + if (!preempt && prev_state && prev_state == prev->state) { I think the compiler won't optimize `prev_state == prev->state` out because of the smp_mb__after_spinlock() which implies a compiler barrier. Still not sure if it's worth making prev->state accesses a READ_ONCE()? Thanks -- Qais Yousef