linux-kernel - Re: nohz problem with idle time on old hardware

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Date:	Wed, 13 Nov 2013 15:02:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Matthew Whitehead <tedheadster@...il.com>
cc:	john.stultz@...aro.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, mwhitehe@...hat.com
Subject: Re: nohz problem with idle time on old hardware

On Wed, 13 Nov 2013, Matthew Whitehead wrote:

> I was testing the 3.12 kernel on some _old_ hardware and I uncovered a bug.
> It arises when nohz=on and goes away with nohz=off. On a crusty dual Pentium-1 
> system that is completely idle, the sar utility reports 0% idle time on cpu0 
> and 100% idle on cpu1. Cpu0 _should_ also be reporting 100% idle, but instead
> it reports around 75% system time and 25% user time.
> 
> The problem was diagnosed by Steve Rostedt with help from John
> Stultz. The old system declares the dual TSCs unstable, and backs
> down to a timesource of refined-jiffies. Apparently refined-jiffies
> and jiffies are not a usable timesourcefor nohz, but we don't check
> for that case because most modern systems have several reliable
> hardware timesources.

Wrong.
 
> John suggested that we turn off nohz unless a usable hardware timesource is
> present.

nohz already depends on two things:

      1) A reliable clocksource which is valid for highres/nohz

      2) A per cpu clockevent device which supports one shot mode.

and those are evaluated at runtime before we switch into NOHZ mode.

And neither jiffies nor refined-jiffies qualify as valid clocksource.

So there is something else wrong.

Thanks,

	tglx
   
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