Re: [PATCH 10/17] 2.6.17.1 perfmon2 patch for review: PMU context switch

[Andi Kleen]

> 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.

And on the other hand a lot of people especially want idle
accounting too and boot with idle=poll. Your explicit 
code would likely defeat that.

> As you realize, that means 
> that we need to turn off when the idle thread is context switched
> in and turn it back on when it is switched off.

Also x86-64 has idle notifiers for this if you really wanted
to do it properly.

-Andi
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