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=DKIM_INVALID,DKIM_SIGNED, 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 BD4ECC43381 for ; Fri, 15 Mar 2019 15:59:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8D7F721871 for ; Fri, 15 Mar 2019 15:59:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="kxPv+dna" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729422AbfCOP7i (ORCPT ); Fri, 15 Mar 2019 11:59:38 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:36718 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729357AbfCOP7i (ORCPT ); Fri, 15 Mar 2019 11:59:38 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=hjk1n1wEPFZC1bHFPZ6efXQGt8+eSh7bBVOvOmDFVik=; b=kxPv+dnarlN68LM+aa30BcVuQ 8j3zKl0JkdQP0CHIIMNcgGu+UOoAN/Pw8ra7Qk7DsUMNSwTkzoT0PGemkkjQTxsHaE1zUf5vEXyR3 Gwxkujuo5WGOqZCzqshxGjuvUNx0cfd26Td0DtI3S+yrwsmB3vRyO8ISEAc0RHCmDJaX1H9f8/mxZ KWQySyUYxIiufNNhqDJsGla2TLRW1zlEuJ1MdiVDXyT6SGIksR6O6o6NO3pSkeqtrwEIqX36UgLzo KLiJ9RdaHgVaXbDWBAvRMoXa8qny9JW0D30AxvCXOcwzGt7Rur+kFpq/GaLYXFoH2KB33MUAD3VI1 SqNd33oNw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1h4pFA-00018S-9q; Fri, 15 Mar 2019 15:59:36 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id C5A2321422949; Fri, 15 Mar 2019 16:59:33 +0100 (CET) Date: Fri, 15 Mar 2019 16:59:33 +0100 From: Peter Zijlstra To: Phil Auld 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: <20190315155933.GY5996@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190315135124.GC27131@pauld.bos.csb> User-Agent: Mutt/1.10.1 (2018-07-13) 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 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.