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=-11.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, 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 1BF59C07E9B for ; Sun, 11 Jul 2021 12:58:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F3217611C1 for ; Sun, 11 Jul 2021 12:58:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232828AbhGKNAw (ORCPT ); Sun, 11 Jul 2021 09:00:52 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:42148 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232730AbhGKNAt (ORCPT ); Sun, 11 Jul 2021 09:00:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626008282; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=WORBSxsfSr261I2DQ2PRKCCN1jy9tToIFkjW7F8syRs=; b=UKCWET8RbbeO7OGfTTfbZzx8frZzz+TH9NPLSKQa3deYiE8guUolTeZLnuFzRhIF6B9RvC zMmONXHjFCM33hu3SXrmvxKfaza7dU2G8JZ7eknqSB+4jCnT46o8jlq/5GkulFL+EHQ9Xi yfacDovJiDfCPJiFPXfprvvH26aGK44= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-428-N1k4NAUePo2zegp9TNIeeA-1; Sun, 11 Jul 2021 08:58:01 -0400 X-MC-Unique: N1k4NAUePo2zegp9TNIeeA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 594E8801B0C; Sun, 11 Jul 2021 12:58:00 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.8.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 819FB60C82; Sun, 11 Jul 2021 12:57:51 +0000 (UTC) Date: Sun, 11 Jul 2021 08:57:49 -0400 From: Phil Auld To: Peter Zijlstra 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: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 09, 2021 at 01:38:20PM +0200 Peter Zijlstra wrote: > 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? > No and no. It was originally a READ_ONCE which didn't compile. I was trying to pair them up but did think that was overkill. I'll try that as soon as I get back to it. Thanks, Phil --