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 <tedheads...@gmail.com> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
