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

[Stephane Eranian]

Andi,

On Fri, Jun 30, 2006 at 03:41:22PM +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'. 
> 

The exclude-idle feature is an option you select when you create
your cpu-wide session. By default, it is off.

> 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.
> 
Yes, but that may not necessarily be troe of all architectures.
At least with the option, the interfaces provides some guarantee.

> 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.
> 
That looks like a useful feature I could leverage but why is it just
on x86-64 at the moment?

-- 
-Stephane
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