Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [PATCH 10/17] 2.6.17.1 perfmon2 patch for review: PMU context switch | Date | Fri, 30 Jun 2006 20:42:23 +0200 |
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On Friday 30 June 2006 20:33, Chuck Ebbert wrote: > In-Reply-To: <200606301541.22928.ak@suse.de> > > On Fri, 30 Jun 2006 15:41:22 +0200, Andi Kleen wrote: > > > > So why do we need care about context switch in cpu-wide mode? > > > It is because we support a mode where the idle thread is excluded > > > from cpu-wide monitoring. This is very useful to distinguish > > > 'useful kernel work' from 'idle'. > > > > I don't quite see the point because on x86 the PMU doesn't run > > during C states anyways. So you get idle excluded automatically. > > Looks like it does run:
I'm pretty sure it doesn't. You can see it by watching the frequency of the perfctr mode NMI watchdog in /proc/interrupts under different loads.
When the system is idle the frequency goes down and increases when the system is busy. I also got confirmation of this behaviour from both Intel and AMD. C states > 0 are not supposed to run the performance counters.
Are you sure you didn't boot with poll=idle? Otherwise something must be wrong with your measurements.
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