Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750774AbaLOMMp (ORCPT ); Mon, 15 Dec 2014 07:12:45 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:34547 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750711AbaLOMMn (ORCPT ); Mon, 15 Dec 2014 07:12:43 -0500 Date: Mon, 15 Dec 2014 13:12:27 +0100 From: Peter Zijlstra To: Ingo Molnar Cc: Sasha Levin , LKML , Dave Jones , Andrey Ryabinin , Linus Torvalds Subject: Re: sched: odd values for effective load calculations Message-ID: <20141215121227.GZ29390@twins.programming.kicks-ass.net> References: <547E42F7.5070105@gmail.com> <20141213083012.GH32572@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141213083012.GH32572@gmail.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sorry for the long delay, I was out for a few weeks due to having become a dad for the second time. On Sat, Dec 13, 2014 at 09:30:12AM +0100, Ingo Molnar wrote: > * Sasha Levin wrote: > > > Hi all, > > > > I was fuzzing with trinity inside a KVM tools guest, running the latest -next > > kernel along with the undefined behaviour sanitizer patch, and hit the following: > > > > [ 787.894288] ================================================================================ > > [ 787.897074] UBSan: Undefined behaviour in kernel/sched/fair.c:4541:17 > > [ 787.898981] signed integer overflow: > > [ 787.900066] 361516561629678 * 101500 cannot be represented in type 'long long int' So that's: this_eff_load *= this_load + effective_load(tg, this_cpu, weight, weight); Going by the numbers the 101500 must be 'this_eff_load', 100 * ~1024 makes that. Which makes the rhs 'large'. Do you have CONFIG_FAIR_GROUP_SCHED enabled? If so, what kind of cgroup hierarchy are you using? In any case, bit sad this doesn't have a register dump included :/ Is this easy to reproduce or something that happened once? > > The values for effective load seem a bit off (and are overflowing!). > > It definitely looks like a bug in SMP load balancing! Yeah, although theoretically (and somewhat practical) this can be triggered in more places if you manage to run up the 'weight' with enough tasks. That said, it should at worst result in 'funny' balancing behaviour, not anything else. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/