linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Tue, 2 Dec 2014 08:33:53 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	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

On Tue, Dec 2, 2014 at 6:13 AM, Mike Galbraith <umgwanakikbuti@...il.com> wrote:
>
> The bean counting problem below can contribute.
>
> https://lkml.org/lkml/2014/3/30/7

Hmm. That never got applied. I didn't apply it originally because of
timing and wanting clarifications, but apparently it never made it
into the -tip tree either.

Ingo, PeterZ - comments?

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.

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.

At the same time, the whole "incapacitated by the rt throttle long
enough for the hard lockup detector to trigger" commentary about that
skip_clock_update issue does make me go "Hmmm..". It would certainly
explain Dave's incomprehensible watchdog messages..

               Linus
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