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

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Date:	Wed, 13 Nov 2013 06:39:27 -0500
From:	Matthew Whitehead <tedheadster@...il.com>
To:	john.stultz@...aro.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Cc:	rostedt@...dmis.org, mwhitehe@...hat.com
Subject: nohz problem with idle time on old hardware

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.

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

- Matthew Whitehead <tedheadster@...il.com>

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