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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 7E787C43381 for ; Fri, 15 Mar 2019 16:19:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 57E6D218A1 for ; Fri, 15 Mar 2019 16:19:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729571AbfCOQTc (ORCPT ); Fri, 15 Mar 2019 12:19:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56268 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726632AbfCOQTb (ORCPT ); Fri, 15 Mar 2019 12:19:31 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A7F6A3086202; Fri, 15 Mar 2019 16:19:31 +0000 (UTC) Received: from pauld.bos.csb (dhcp-17-51.bos.redhat.com [10.18.17.51]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 27182600C2; Fri, 15 Mar 2019 16:19:31 +0000 (UTC) Date: Fri, 15 Mar 2019 12:19:29 -0400 From: Phil Auld To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Ben Segall , Ingo Molnar Subject: Re: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup Message-ID: <20190315161929.GH27131@pauld.bos.csb> References: <20190313150826.16862-1-pauld@redhat.com> <20190315101150.GV5996@hirez.programming.kicks-ass.net> <20190315103357.GC6521@hirez.programming.kicks-ass.net> <20190315135124.GC27131@pauld.bos.csb> <20190315155933.GY5996@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190315155933.GY5996@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.42]); Fri, 15 Mar 2019 16:19:31 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 15, 2019 at 04:59:33PM +0100 Peter Zijlstra wrote: > On Fri, Mar 15, 2019 at 09:51:25AM -0400, Phil Auld wrote: > > On Fri, Mar 15, 2019 at 11:33:57AM +0100 Peter Zijlstra wrote: > > > On Fri, Mar 15, 2019 at 11:11:50AM +0100, Peter Zijlstra wrote: > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index ea74d43924b2..b71557be6b42 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -4885,6 +4885,8 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer) > > > > return HRTIMER_NORESTART; > > > > } > > > > > > > > +extern const u64 max_cfs_quota_period; > > > > + > > > > static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer) > > > > { > > > > struct cfs_bandwidth *cfs_b = > > > > @@ -4892,6 +4894,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer) > > > > unsigned long flags; > > > > int overrun; > > > > int idle = 0; > > > > + int count = 0; > > > > > > > > raw_spin_lock_irqsave(&cfs_b->lock, flags); > > > > for (;;) { > > > > @@ -4899,6 +4902,28 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer) > > > > if (!overrun) > > > > break; > > > > > > > > + if (++count > 3) { > > > > + u64 new, old = ktime_to_ns(cfs_b->period); > > > > + > > > > + new = (old * 147) / 128; /* ~115% */ > > > > + new = min(new, max_cfs_quota_period); > > > > > > Also, we can still engineer things to come unstuck; if we explicitly > > > configure period at 1e9 and then set a really small quota and then > > > create this insane amount of cgroups you have.. > > > > > > this code has no room to manouvre left. > > > > > > Do we want to do anything about that? Or leave it as is, don't do that > > > then? > > > > > > > If the period is 1s it would be hard to make this loop fire repeatedly. I don't think > > it's that dependent on the quota other than getting some rqs throttled. The small quota > > would also mean fewer of them would get unthrottled per distribute call. You'd probably > > need _significantly_ more cgroups than my insane 2500 to hit it. > > > > Right now it settles out with a new period of ~12-15ms. So ~200,000 cgroups? > > > > Ben and I talked a little about this in another thread. I think hitting this is enough of > > an edge case that this approach will make the problem go away. The only alternative we > > came up with to reduce the time taken in unthrottle involved a fair bit of complexity > > added to the every day code paths. And might not help if the children all had their > > own quota/period settings active. > > Ah right. I forgot that part. And yes, I remember what was proposed to > avoid the tree walk, that wouldn't have been pretty. I'm glad I was not the only one who was not excited by that :) --