linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Mon, 5 Jan 2015 17:25:31 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Dave Jones <davej@...emonkey.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>, Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Anvin <hpa@...ux.intel.com>
Subject: Re: frequent lockups in 3.18rc4

On Mon, Jan 5, 2015 at 5:17 PM, John Stultz <john.stultz@...aro.org> wrote:
>
> Anyway, It may be worth keeping the 50% margin (and dropping the 12%
> reduction to simplify things)

Again, the 50% margin is only on the multiplication overflow. Not on the mask.

So it won't do anything at all for the case we actually care about,
namely a broken HPET, afaik.

I'd much rather limit to 50% of the mask too.

Also, why do we actually play games with ilog2 for that overflow
calculation? It seems pointless. This is for the setup code, doing a
real division there would seem to be a whole lot more straightforward,
and not need that big comment. And there's no performance issue. Am I
missing something?

> I've also got a capping patch that I'm testing that keeps time reads
> from passing that interval. The only thing I'm really cautious about
> with that change is that we have to make sure the hrtimer that
> triggers update_wall_clock is always set to expire within that cap (I
> need to review it again) or else we'll hang ourselves.

 Yeah, that thing is fragile. And quite possibly part of the problem.

                       Linus
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