Messages in this thread |  | | Date | Wed, 13 Nov 2013 06:39:27 -0500 | From | Matthew Whitehead <> | Subject | nohz problem with idle time on old hardware |
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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@gmail.com>
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