linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Sat, 13 Dec 2014 09:11:40 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Mason <clm@...com>,
	Dâniel Fraga <fragabr@...il.com>,
	Dave Jones <davej@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4


* Mike Galbraith <umgwanakikbuti@...il.com> wrote:

> On Tue, 2014-12-02 at 08:33 -0800, Linus Torvalds wrote:
> 
> > Looking again at that patch (the commit message still doesn't strike
> > me as wonderfully explanatory :^) makes me worry, though.
> > 
> > Is that
> > 
> >         if (rq->skip_clock_update-- > 0)
> >                 return;
> > 
> > really right? If skip_clock_update was zero (normal), it now gets set
> > to -1, which has its own specific meaning (see "force clock update"
> > comment in kernel/sched/rt.c). Is that intentional? That seems insane.
> 
> Yeah, it was intentional.  Least lines.
> 
> > Or should it be
> > 
> >         if (rq->skip_clock_update > 0) {
> >                 rq->skip_clock_update = 0;
> >                 return;
> >         }
> > 
> > or what? Maybe there was a reason the patch never got applied even to -tip.
> 
> Peterz was looking at corner case proofing the thing.  Saving those
> cycles has been entirely too annoying.
> 
> https://lkml.org/lkml/2014/4/8/295

Hm, so that discussion died with:

  https://lkml.org/lkml/2014/4/8/343

Did you ever get around to trying Peter's patch?

But ... I've yet to see rq_clock problems cause actual lockups. 
That's the main problem we have with its (un)robustness and why 
Peter created that rq_clock debug facility: bugs there cause 
latencies but no easily actionable symptoms, which are much 
harder to debug.

Thanks,

	Ingo
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