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Date: Tue, 2 Dec 2014 08:33:53 -0800
Message-ID: <CA+55aFw8smHBw9HiCiYL_ohkULLeunWo6qfayM19zhF1hKAxXg@mail.gmail.com>
Subject: Re: frequent lockups in 3.18rc4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mike Galbraith <umgwanakikbuti@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc: Chris Mason <clm@fb.com>,
        =?UTF-8?Q?D=C3=A2niel_Fraga?= <fragabr@gmail.com>,
        Dave Jones <davej@redhat.com>, Sasha Levin <sasha.levin@oracle.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Tue, Dec 2, 2014 at 6:13 AM, Mike Galbraith <umgwanakikbuti@gmail.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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