linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <1417542432.3924.39.camel@maggy.simpson.net>
Date:	Tue, 02 Dec 2014 10:47:12 -0700
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.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

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

	-Mike

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