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Subject: Re: frequent lockups in 3.18rc4
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Chris Mason <clm@fb.com>,
        =?ISO-8859-1?Q?D=E2niel?= Fraga <fragabr@gmail.com>,
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        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date: Sat, 13 Dec 2014 10:57:42 +0100
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On Sat, 2014-12-13 at 09:11 +0100, Ingo Molnar wrote: 
> * Mike Galbraith <umgwanakikbuti@gmail.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?

I couldn't plug it into the production -ENOBOOT IO beasts from hell, but
did run it on my little desktop a bit.
> 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.

If watchdog gets credit for zillion disk detection time, it can end up
throttled for what is effectively forever.

-Mike