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=-10.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 71C4DC433E2 for ; Tue, 1 Sep 2020 17:30:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4EF5120BED for ; Tue, 1 Sep 2020 17:30:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="QA40R1vz" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729049AbgIARa4 (ORCPT ); Tue, 1 Sep 2020 13:30:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44706 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726244AbgIARaz (ORCPT ); Tue, 1 Sep 2020 13:30:55 -0400 Received: from mail-qt1-x843.google.com (mail-qt1-x843.google.com [IPv6:2607:f8b0:4864:20::843]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0978BC061244 for ; Tue, 1 Sep 2020 10:30:55 -0700 (PDT) Received: by mail-qt1-x843.google.com with SMTP id 60so1519090qtc.9 for ; Tue, 01 Sep 2020 10:30:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to; bh=Dv3VdlkLKfSzN/kpT7a37/cDiEE7QKzbn4dVlTcp+8E=; b=QA40R1vzfssiArrSOjzgXirThCZeDBEprFlZcdiHbR5lBfh7efubugybosUZ5DCAxv N6kKKp2/U6wQWk1vYx2h3Etc+uzTKRn0InsDl+vAQK8ai/njhpURcXTI831nkjxwyxX0 qOnnlhU5hthTKIRUE0j8m0v5OscwgsZstK1cE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=Dv3VdlkLKfSzN/kpT7a37/cDiEE7QKzbn4dVlTcp+8E=; b=kfLwnTg2lvKisa5EIs0EbK2XMG3EYr9SErDu5nmrpcG8Wf4Z3Qc8PRYWrZZmtj/z+O uuWqYk5g4SEsOpWOjTUUurj0UPSjTAeaWaQTW7BRYhHXUKTf2K5Tb6NuBGSPpbxUsRLv gapmIiGiz/1INCVGVM3tbPLWf0sLnitd4gU9kOxjVsiEamomRC/O3j+hIk8iXXu2ePkq A4nCz8kwGGmr7HIdfLxt3sKEJqaLEse4xs2uJnay5Tf2uwlnXuschT8S4Eha81AE4vmI i53Wb1VN66+RvbwTjrG0jMax/gf6bwV4dCppb4eAdhgO0Drn0djTj502t1ZrljJu2VEV gG5w== X-Gm-Message-State: AOAM533OT8sMZ4nOZ6FSrDAsdcuOOez6e8VyUat0ajJoX34fljVIEEJT 2oDGCziQ9m4BJ0GTFU6G2iHO1IZWEj4BTA== X-Google-Smtp-Source: ABdhPJwgbP34OXWYQ6N1WOoOKLxJN0GBQCM4C7A60o92pxYcFtyzbdXcMZXlecfTB35mSD7s6zik+g== X-Received: by 2002:aed:2d62:: with SMTP id h89mr2928823qtd.193.1598981454071; Tue, 01 Sep 2020 10:30:54 -0700 (PDT) Received: from localhost ([2620:15c:6:12:cad3:ffff:feb3:bd59]) by smtp.gmail.com with ESMTPSA id y73sm2420586qkb.23.2020.09.01.10.30.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Sep 2020 10:30:53 -0700 (PDT) Date: Tue, 1 Sep 2020 13:30:52 -0400 From: Joel Fernandes To: Vineeth Pillai Cc: peterz@infradead.org, Julien Desfossez , Tim Chen , Aaron Lu , Aubrey Li , Dhaval Giani , Chris Hyser , Nishanth Aravamudan , mingo@kernel.org, tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com, Phil Auld , Valentin Schneider , Mel Gorman , Pawan Gupta , Paolo Bonzini , vineeth@bitbyteword.org, Chen Yu , Christian Brauner , Agata Gruza , Antonio Gomez Iglesias , graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com, rostedt@goodmis.org, derkling@google.com, benbjiang@tencent.com, Vineeth Remanan Pillai , Aaron Lu Subject: Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling. Message-ID: <20200901173052.GA1703315@google.com> References: <20200828205154.GB29142@worktop.programming.kicks-ass.net> <381e6ea5-a48c-9882-4c0d-49cfa92d21cc@linux.microsoft.com> <20200829074719.GJ1362448@hirez.programming.kicks-ass.net> <20200901051014.GA3993517@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Vineeth, On Tue, Sep 01, 2020 at 08:34:23AM -0400, Vineeth Pillai wrote: > Hi Joel, > > On 9/1/20 1:10 AM, Joel Fernandes wrote: > > 3. The 'Rescheduling siblings' loop of pick_next_task() is quite fragile. It > > calls various functions on rq->core_pick which could very well be NULL because: > > An online sibling might have gone offline before a task could be picked for it, > > or it might be offline but later happen to come online, but its too late and > > nothing was picked for it. Just ignore the siblings for which nothing could be > > picked. This avoids any crashes that may occur in this loop that assume > > rq->core_pick is not NULL. > > > > Signed-off-by: Joel Fernandes (Google) > I like this idea, its much simpler :-) Thanks. > > --- > > kernel/sched/core.c | 24 +++++++++++++++++++++--- > > 1 file changed, 21 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 717122a3dca1..4966e9f14f39 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -4610,13 +4610,24 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > > if (!sched_core_enabled(rq)) > > return __pick_next_task(rq, prev, rf); > > + cpu = cpu_of(rq); > > + > > + /* Stopper task is switching into idle, no need core-wide selection. */ > > I think we can come here when hotplug thread is scheduled during online, but > mask is not yet updated. Probably can add it with this comment as well. > I don't see how that is possible. Because the cpuhp threads run during the CPU onlining process, the boot thread for the CPU coming online would have already updated the mask. > > + if (cpu_is_offline(cpu)) > > + return __pick_next_task(rq, prev, rf); > > + > We would need reset core_pick here I think. Something like >     if (cpu_is_offline(cpu)) { >         rq->core_pick = NULL; >         return __pick_next_task(rq, prev, rf); >     } > > Without this we can end up in a crash like this: > 1. Sibling of this cpu picks a task (rq_i->core_pick) and this cpu goes >     offline soon after. > 2. Before this cpu comes online, sibling goes through another pick loop >     and before its IPI loop, this cpu comes online and we get an IPI. > 3. So when this cpu gets into schedule, we have core_pick set and >     core_pick_seq != core_sched_seq. So we enter the fast path. But >     core_pick might no longer in this runqueue. > > So, to protect this, we should reset core_pick I think. I have seen this > crash > occasionally. Ok, done. > > /* > > * If there were no {en,de}queues since we picked (IOW, the task > > * pointers are all still valid), and we haven't scheduled the last > > * pick yet, do so now. > > + * > > + * rq->core_pick can be NULL if no selection was made for a CPU because > > + * it was either offline or went offline during a sibling's core-wide > > + * selection. In this case, do a core-wide selection. > > */ > > if (rq->core->core_pick_seq == rq->core->core_task_seq && > > - rq->core->core_pick_seq != rq->core_sched_seq) { > > + rq->core->core_pick_seq != rq->core_sched_seq && > > + !rq->core_pick) { > Should this check be reversed? I mean, we should enter the fastpath if > we have rq->core_pick is set right? Done. Sorry my testing did not catch it, but it eventually caused a problem after several hours of the stress test so I'd have eventually caught it. > Another unrelated, but related note :-) > Besides this, I think we need to retain on more change from the previous > patch. We would need to make core_pick_seq per sibling instead of per > core. Having it per core might lead to unfairness. For eg: When a cpu > sees that its sibling's core_pick is the one which is already running, it > will not send IPI. but core_pick remains set and core->core_pick_seq is > incremented. Now if the sibling is preempted due to a high priority task Then don't keep the core_pick set then. If you don't send it IPI and if core_pick is already running, then NULL it already. I don't know why we add to more corner cases by making assumptions. We have enough open issues that are not hotplug related. Here's my suggestion : 1. Keep the ideas consistent, forget about the exact code currently written and just understand the pick_seq is for siblings knowing that something was picked for the whole core. So if their pick_seq != sched_seq, then they have to pick what was selected. 2. If core_pick should be NULL, then NULL it in some path. If you keep some core_pick and you increment pick_seq, then you are automatically asking the sibling to pick that task up then next time it enters schedule(). See if [1] will work? Note that, we have added logic in this patch that does a full selection if rq->core_pick == NULL. > or its time slice expired, it enters schedule. But it goes to fast path and > selects the running task there by starving the high priority task. Having > the core_pick_seq per sibling will avoid this. It might also help in some > hotplug corner cases as well. That can be a separate patch IMHO. It has nothing to do with stability/crashing of concurrent and rather infrequent CPU hotplug operations. Also, Peter said pick_seq is for core-wide picking. If you want to add another semantic, then maybe add another counter which has a separate meaning and justify why you are adding it. thanks, - Joel [1] ---8<----------------------- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7728ca7f6bb2..7a03b609e3b7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4793,6 +4793,8 @@ next_class:; if (rq_i->curr != rq_i->core_pick) resched_curr(rq_i); + else + rq_i->core_pick = NULL; /* Did we break L1TF mitigation requirements? */ WARN_ON_ONCE(!cookie_match(next, rq_i->core_pick));