linux-kernel - Re: sched: odd values for effective load calculations

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Date:	Thu, 18 Dec 2014 10:10:51 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: sched: odd values for effective load calculations

On Tue, Dec 16, 2014 at 12:29:28AM -0500, Sasha Levin wrote:
> On 12/15/2014 08:14 AM, Peter Zijlstra wrote:
> >>>> [  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 :/
> > Hmm, I was hoping to be able to see if it was this_load or the
> > effective_load() result being silly large, but going by the ASM output
> > of my compiler this isn't going to be available in registers, its all
> > stack spills.
> > 
> > Pinning my hopes on that reproducability thing :/
> 
> Okay, yeah, it's very reproducible. I've added:
> 

Hi Sasha,

I tried to reproduce this overflow, but got not luck (the trinity has been
running in a KVM VM for an hour)...

The trinity used is v1.4, and simply launched as ./trinity.

Could you detail your setup and procedure?

Thanks,
Yuyang
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