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 B72E0C07E9B for ; Fri, 9 Jul 2021 11:38:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 983CC613BA for ; Fri, 9 Jul 2021 11:38:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231210AbhGILlO (ORCPT ); Fri, 9 Jul 2021 07:41:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49246 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231149AbhGILlN (ORCPT ); Fri, 9 Jul 2021 07:41:13 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 88867C0613DD; Fri, 9 Jul 2021 04:38:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=43Lf+zzmrrH5VsxADYa58paCbsgJPKuh8uE7fsHJkIM=; b=bzOSMrlT2MuULTiykTbSEevQBy BqdAfWy86bLpmFBMKGulUid+AfAXjqmMFUE/hTY5FnWCpL/KlzmWPb2s5LfXZ5sLT3ZaHjEBKUIGH WEFtmOyZMHkNLohpfXlvj96GX6yeqXuqnKF+xXYezmnPc8aF08kg88bAKfTIkU1a7uiQtdRtdNe/M GAQeb2UhnWu8btSaep0kSrNWq1zCNnoN4Dnne9bBDZU01gOY7vrSdVBr8m5QS6rzo5jfE6ZeSsifk DAVz57tVGstU+gkOu9Qoii2LKaHW1Auuc65jH5sNLdUgFTZ9TcKwV25+880bZVD2BEleCpntOv9Fm yiqoAxPg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1m1opq-00G47Y-On; Fri, 09 Jul 2021 11:38:23 +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 154D130022B; Fri, 9 Jul 2021 13:38:21 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 02EA121BE8C9E; Fri, 9 Jul 2021 13:38:20 +0200 (CEST) Date: Fri, 9 Jul 2021 13:38:20 +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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 08, 2021 at 09:25:45AM -0400, Phil Auld wrote: > Hi Peter, > > On Thu, Jul 08, 2021 at 09:26:26AM +0200 Peter Zijlstra wrote: > > 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. > > > > Yes, it is needed. We've got idle power systems with load average of 530.21. > Calc_load_tasks is 530, and the sum of both nr_uninterruptible and > calc_load_active across all the runqueues is 530. Basically monotonically > non-decreasing load average. With the patch this no longer happens. Have you tried without the rmb here? Do you really need both barriers?