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* nohz problem with idle time on old hardware
@ 2013-11-13 11:39 Matthew Whitehead
  2013-11-13 14:02 ` Thomas Gleixner
  0 siblings, 1 reply; 29+ messages in thread
From: Matthew Whitehead @ 2013-11-13 11:39 UTC (permalink / raw)
  To: john.stultz, tglx, linux-kernel; +Cc: rostedt, mwhitehe

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>


^ permalink raw reply	[flat|nested] 29+ messages in thread

end of thread, other threads:[~2014-04-09 16:15 UTC | newest]

Thread overview: 29+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2013-11-13 11:39 nohz problem with idle time on old hardware Matthew Whitehead
2013-11-13 14:02 ` Thomas Gleixner
2013-11-13 15:21   ` Steven Rostedt
2013-11-13 15:31     ` Steven Rostedt
2013-11-13 15:50       ` Thomas Gleixner
2013-11-13 16:17         ` Paul E. McKenney
2013-11-13 15:57       ` Steven Rostedt
2013-11-13 16:07         ` Thomas Gleixner
2013-11-13 16:12           ` Steven Rostedt
2013-11-13 16:18             ` Paul E. McKenney
2013-11-13 16:23               ` Steven Rostedt
2013-11-13 16:35                 ` Paul E. McKenney
2013-11-13 20:01                   ` Thomas Gleixner
2013-11-13 20:07                     ` Steven Rostedt
2013-11-13 21:49                       ` Matthew Whitehead
2013-11-18 22:44                     ` Paul E. McKenney
2013-11-19 18:07                     ` [tip:timers/urgent] NOHZ: Check for nohz active instead of nohz enabled tip-bot for Thomas Gleixner
2014-04-09 13:51                     ` nohz problem with idle time on old hardware Viresh Kumar
2014-04-09 14:31                       ` Steven Rostedt
2014-04-09 15:20                         ` Viresh Kumar
2014-04-09 15:29                           ` Steven Rostedt
2014-04-09 15:31                             ` Steven Rostedt
2014-04-09 15:34                               ` Viresh Kumar
2014-04-09 15:39                               ` Steven Rostedt
2014-04-09 15:56                                 ` Viresh Kumar
2014-04-09 16:15                                   ` Steven Rostedt
2014-04-09 15:11                       ` Frederic Weisbecker
2013-11-13 16:21           ` Thomas Gleixner
2013-11-13 16:31             ` Steven Rostedt

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