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=-11.0 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=unavailable 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 109DCC07E99 for ; Thu, 8 Jul 2021 07:27:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E3C4561CE0 for ; Thu, 8 Jul 2021 07:26:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229966AbhGHH3k (ORCPT ); Thu, 8 Jul 2021 03:29:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39386 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229819AbhGHH3j (ORCPT ); Thu, 8 Jul 2021 03:29:39 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E8D3EC061574; Thu, 8 Jul 2021 00:26:57 -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=vl5MbRdGwmlZh7qhtnufnGU16TVZ9C/OsTo2b3jA4CM=; b=IK5y5r7nAKeB3x4HLrBMKfQ2Dm Zob9xcDtZGLg/wdtxqqXN9bQSGYk4Sbxr2DMEKtbCsTvN6anqUayPDYVyyso6a0ar8GjGVCiBu480 0TGlwVrvJimpl0UeiBKkz5CaaJo2LtRmHZXou+vWcTBVY7XDm1PyO9TSRmtPGSFtFxlfhUvuf/2kJ f2PMD134yqCjxSmd0pgM3PjAoj2YOEyIUiyz4+Zts4g76uBREqUggXtbbH5YMnIsrAMJJvALVfNCa UKk0s/pkMpOtS45zjDnJ+GjRkwzPc7V5d8XQ8To4/eDK5Z5fhK7fGrX7HOOfAjx2Xf4xAEWBnYHw0 zsbDMTZQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1m1OQV-00DBl7-Es; Thu, 08 Jul 2021 07:26:31 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 4A8F630007E; Thu, 8 Jul 2021 09:26:26 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 2A8F82D16D2AC; Thu, 8 Jul 2021 09:26:26 +0200 (CEST) Date: Thu, 8 Jul 2021 09:26:26 +0200 From: Peter Zijlstra To: Phil Auld Cc: linux-kernel , Waiman Long , Ingo Molnar , Juri Lelli , Vincent Guittot , stable@vger.kernel.org Subject: Re: [PATCH] sched: Fix nr_uninterruptible race causing increasing load average Message-ID: References: <20210707190457.60521-1-pauld@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210707190457.60521-1-pauld@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 07, 2021 at 03:04:57PM -0400, Phil Auld wrote: > On systems with weaker memory ordering (e.g. power) commit dbfb089d360b > ("sched: Fix loadavg accounting race") causes increasing values of load > average (via rq->calc_load_active and calc_load_tasks) due to the wakeup > CPU not always seeing the write to task->sched_contributes_to_load in > __schedule(). Missing that we fail to decrement nr_uninterruptible when > waking up a task which incremented nr_uninterruptible when it slept. > > The rq->lock serialization is insufficient across different rq->locks. > > Add smp_wmb() to schedule and smp_rmb() before the read in > ttwu_do_activate(). > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 4ca80df205ce..ced7074716eb 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2992,6 +2992,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags, > > lockdep_assert_held(&rq->lock); > > + /* Pairs with smp_wmb in __schedule() */ > + smp_rmb(); > if (p->sched_contributes_to_load) > rq->nr_uninterruptible--; > Is this really needed ?! (this question is a big fat clue the comment is insufficient). AFAICT try_to_wake_up() has a LOAD-ACQUIRE on p->on_rq and hence the p->sched_contributed_to_load must already happen after. > @@ -5084,6 +5086,11 @@ static void __sched notrace __schedule(bool preempt) > !(prev_state & TASK_NOLOAD) && > !(prev->flags & PF_FROZEN); > > + /* > + * Make sure the previous write is ordered before p->on_rq etc so > + * that it is visible to other cpus in the wakeup path (ttwu_do_activate()). > + */ > + smp_wmb(); > if (prev->sched_contributes_to_load) > rq->nr_uninterruptible++; That comment is terrible, look at all the other barrier comments around there for clues; in effect you're worrying about: p->sched_contributes_to_load = X R1 = p->on_rq WMB RMB p->on_rq = Y R2 = p->sched_contributes_to_load Right? Bah bah bah.. I so detest having to add barriers here for silly accounting. Let me think about this a little.