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 ECDBEC07E96 for ; Sun, 11 Jul 2021 13:20:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CFEC9611CE for ; Sun, 11 Jul 2021 13:20:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232840AbhGKNXG (ORCPT ); Sun, 11 Jul 2021 09:23:06 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:59647 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232756AbhGKNXE (ORCPT ); Sun, 11 Jul 2021 09:23:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626009617; 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=ii+7ydEQcrkoc4EY60mMuxmS0HQ1FH1ba93hoFgQD7Q=; b=bEDNsNxkqs1rZ2ZWZL9IAuqZ6oAVCM+pzO4DGviw3FIdGuWymqkC9SQ0ExcvCgtgxCKCt3 EBtkUbyByScdQpHevvz39kapt598bM5PY7jjhcY71F2Q7RwPHdz+So2VFT7VLZxVxWMTJr 6ezDHmIiqvaEnhPhu9iH9bRsrltRFMU= 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-507-LEf1oYJsMVipokhaqNcy_w-1; Sun, 11 Jul 2021 09:20:16 -0400 X-MC-Unique: LEf1oYJsMVipokhaqNcy_w-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A7AAE801107; Sun, 11 Jul 2021 13:20:14 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.8.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EED2B5C22B; Sun, 11 Jul 2021 13:19:57 +0000 (UTC) Date: Sun, 11 Jul 2021 09:19:56 -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 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, On Fri, Jul 09, 2021 at 02:57:10PM +0200 Peter Zijlstra wrote: > On Thu, Jul 08, 2021 at 10:54:58AM -0400, Phil Auld wrote: > > Sorry... I don't have a nice diagram. I'm still looking at what all those > > macros actually mean on the various architectures. > > Don't worry about other architectures, lets focus on Power, because > that's the case where you can reprouce funnies. Now Power only has 2 > barrier ops (not quite true, but close enough for all this): > > - SYNC is the full barrier > > - LWSYNC is a TSO like barrier > > Pretty much everything (LOAD-ACQUIRE, STORE-RELEASE, WMB, RMB) uses > LWSYNC. Only MB result in SYNC. > > Power is 'funny' because their spinlocks are weaker than everybody > else's, but AFAICT that doesn't seem relevant here. > Thanks. > > Using what you have above I get the same thing. It looks like it should be > > ordered but in practice it's not, and ordering it "more" as I did in the > > patch, fixes it. > > And you're running Linus' tree, not some franken-kernel from RHT, right? > As asked in that other email, can you try with just the WMB added? I > really don't believe that RMB you added can make a difference. So, no. Right now the reproducer is on the franken-kernel :( As far as I can tell the relevant code paths (schedule, barriers, wakeup etc) are all current and the same. I traced through your diagram and it all matches exactly. I have a suspicion that Linus's tree may hide it. I believe this is tickled by NFS io, which I _think_ is effected by the unboud workqueue changes that may make it less likely to do the wakeup on a different cpu. But that's just speculation. The issue is that the systems under test here are in a partner's lab to which I have no direct access. I will try to get an upstream build on there, if possible, as soon as I can. > > Also, can you try with TTWU_QUEUE disabled (without any additional > barriers added), that simplifies the wakeup path a lot. > Will do. > > Is it possible that the bit field is causing some of the assumptions about > > ordering in those various macros to be off? > > *should* not matter... > > prev->sched_contributes_to_load = X; > > smp_store_release(&prev->on_cpu, 0); > asm("LWSYNC" : : : "memory"); > WRITE_ONCE(prev->on_cpu, 0); > > due to that memory clobber, the compiler must emit whatever stores are > required for the bitfield prior to the LWSYNC. > > > I notice in all the comments about smp_mb__after_spinlock etc, it's always > > WRITE_ONCE/READ_ONCE on the variables in question but we can't do that with > > the bit field. > > Yeah, but both ->on_rq and ->sched_contributes_to_load are 'normal' > stores. That said, given that ttwu() does a READ_ONCE() on ->on_rq, we > should match that with WRITE_ONCE()... > > So I think we should do the below, but I don't believe it'll make a > difference. Let me stare more. > I'm out of the office for the next week+ so don't stare to hard. I'll try to get the tests you asked for as soon as I get back in the (home) office. I'm not sure the below will make a difference either, but will try it too. Thanks again for the help. And sorry for the timing. Cheers, Phil > --- > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index ca9a523c9a6c..da93551b298d 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1973,12 +1973,12 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags) > { > enqueue_task(rq, p, flags); > > - p->on_rq = TASK_ON_RQ_QUEUED; > + WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED); > } > > void deactivate_task(struct rq *rq, struct task_struct *p, int flags) > { > - p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING; > + WRITE_ONCE(p->on_rq, (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING); > > dequeue_task(rq, p, flags); > } > @@ -5662,11 +5662,11 @@ static bool try_steal_cookie(int this, int that) > if (p->core_occupation > dst->idle->core_occupation) > goto next; > > - p->on_rq = TASK_ON_RQ_MIGRATING; > + WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING); > deactivate_task(src, p, 0); > set_task_cpu(p, this); > activate_task(dst, p, 0); > - p->on_rq = TASK_ON_RQ_QUEUED; > + WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED); > > resched_curr(dst); > > --