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=-5.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 9F8E4C388F9 for ; Fri, 23 Oct 2020 19:28:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D52792168B for ; Fri, 23 Oct 2020 19:28:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="WP5vCew0" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753860AbgJWT2G (ORCPT ); Fri, 23 Oct 2020 15:28:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48098 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750070AbgJWT2G (ORCPT ); Fri, 23 Oct 2020 15:28:06 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C3392C0613CE for ; Fri, 23 Oct 2020 12:28:05 -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=SMzYJ4HmuSwXSTmcc18Yf6TgQ0PU6jqXVbmfJ9VGFWo=; b=WP5vCew0NONWkSMzdkjdtkcr4T Vhq55eHkYhRojxEiiX2qq5kBMBcsxKs2PzLHw76oGOhBrEike/fffyASZPpcTPhaxTgajgi3ySwGE 59BzJTi2Qm3yv81iS/cVXLaR3cnKsWnB0xF9NfIG/WqcizZ8/nqxlpKvUnkjjdZmb9loNIeHS2si5 Cp/ImFhyWeEWVJlcV4S6SrVrSZhz5o5UUJ9qarpPibHi856rRrQqGCVAOLMBzK8l/CuI/bvqQnWwe nlpoTAK5T4UWOJYi8WoqgNaX6b1uaoY5hqtL9E3FXXv5i5W1Uw3IELyn0Eid9eMkASePcADwC2G2W LZcpmnKw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kW2iG-0003pH-7N; Fri, 23 Oct 2020 19:26:56 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id C674F98104F; Fri, 23 Oct 2020 21:26:54 +0200 (CEST) Date: Fri, 23 Oct 2020 21:26:54 +0200 From: Peter Zijlstra To: Joel Fernandes Cc: Nishanth Aravamudan , Julien Desfossez , Tim Chen , Vineeth Pillai , Aaron Lu , Aubrey Li , tglx@linutronix.de, linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.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, pjt@google.com, rostedt@goodmis.org, derkling@google.com, benbjiang@tencent.com, Alexandre Chartre , James.Bottomley@hansenpartnership.com, OWeisse@umich.edu, Dhaval Giani , Junaid Shahid , jsbarnes@google.com, chris.hyser@oracle.com, Vineeth Remanan Pillai , Aaron Lu , Aubrey Li , "Paul E. McKenney" , Tim Chen Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling. Message-ID: <20201023192654.GH2974@worktop.programming.kicks-ass.net> References: <20201020014336.2076526-1-joel@joelfernandes.org> <20201020014336.2076526-7-joel@joelfernandes.org> <20201023135129.GS2611@hirez.programming.kicks-ass.net> <20201023135400.GA2651@hirez.programming.kicks-ass.net> <20201023175724.GA3563800@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201023175724.GA3563800@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 23, 2020 at 01:57:24PM -0400, Joel Fernandes wrote: > On Fri, Oct 23, 2020 at 03:54:00PM +0200, Peter Zijlstra wrote: > > On Fri, Oct 23, 2020 at 03:51:29PM +0200, Peter Zijlstra wrote: > > > On Mon, Oct 19, 2020 at 09:43:16PM -0400, Joel Fernandes (Google) wrote: > > > > + /* > > > > + * If this sibling doesn't yet have a suitable task to > > > > + * run; ask for the most elegible task, given the > > > > + * highest priority task already selected for this > > > > + * core. > > > > + */ > > > > + p = pick_task(rq_i, class, max); > > > > + if (!p) { > > > > + /* > > > > + * If there weren't no cookies; we don't need to > > > > + * bother with the other siblings. > > > > + * If the rest of the core is not running a tagged > > > > + * task, i.e. need_sync == 0, and the current CPU > > > > + * which called into the schedule() loop does not > > > > + * have any tasks for this class, skip selecting for > > > > + * other siblings since there's no point. We don't skip > > > > + * for RT/DL because that could make CFS force-idle RT. > > > > + */ > > > > + if (i == cpu && !need_sync && class == &fair_sched_class) > > > > + goto next_class; > > > > + > > > > + continue; > > > > + } > > > > > > I'm failing to understand the class == &fair_sched_class bit. > > The last line in the comment explains it "We don't skip for RT/DL because > that could make CFS force-idle RT.". Well, yes, but it does not explain how this can come about, now does it. > Even if need_sync == false, we need to go look at other CPUs (non-local > CPUs) to see if they could be running RT. > > Say the RQs in a particular core look like this: > Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task. > > rq0 rq1 > CFS1 (tagged) RT1 (not tag) > CFS2 (tagged) > > Say schedule() runs on rq0. Now, it will enter the above loop and > pick_task(RT) will return NULL for 'p'. It will enter the above if() block > and see that need_sync == false and will skip RT entirely. > > The end result of the selection will be (say prio(CFS1) > prio(CFS2)): > rq0 rq1 > CFS1 IDLE > > When it should have selected: > rq0 r1 > IDLE RT > > I saw this issue on real-world usecases in ChromeOS where an RT task gets > constantly force-idled and breaks RT. The "class == &fair_sched_class" bit > cures it. Ah, I see. The thing is, this looses the optimization for a bunch of valid (and arguably common) scenarios. The problem is that the moment we end up selecting a task with a cookie we've invalidated the premise under which we ended up with the selected task. How about this then? --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4709,6 +4709,7 @@ pick_next_task(struct rq *rq, struct tas need_sync = !!rq->core->core_cookie; /* reset state */ +reset: rq->core->core_cookie = 0UL; for_each_cpu(i, smt_mask) { struct rq *rq_i = cpu_rq(i); @@ -4748,14 +4749,8 @@ pick_next_task(struct rq *rq, struct tas /* * If there weren't no cookies; we don't need to * bother with the other siblings. - * If the rest of the core is not running a tagged - * task, i.e. need_sync == 0, and the current CPU - * which called into the schedule() loop does not - * have any tasks for this class, skip selecting for - * other siblings since there's no point. We don't skip - * for RT/DL because that could make CFS force-idle RT. */ - if (i == cpu && !need_sync && !p->core_cookie) + if (i == cpu && !need_sync) goto next_class; continue; @@ -4765,7 +4760,17 @@ pick_next_task(struct rq *rq, struct tas * Optimize the 'normal' case where there aren't any * cookies and we don't need to sync up. */ - if (i == cpu && !need_sync && !p->core_cookie) { + if (i == cpu && !need_sync) { + if (p->core_cookie) { + /* + * This optimization is only valid as + * long as there are no cookies + * involved. + */ + need_sync = true; + goto reset; + } + next = p; goto done; } @@ -4805,7 +4810,6 @@ pick_next_task(struct rq *rq, struct tas */ need_sync = true; } - } } next_class:;