Re: [patch 02/24] perfmon: base code

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 27 Nov 2008, Andi Kleen wrote:

> > Well, where is it checked ? Where is checked whether Oprofile runs or not ?
> 
> That is done using the perfctr reservation. I saw that somewhere in the
> patchkit. The NMI watchdog uses that too.
> 
> > > The Oprofile reservation you see is built on top of the cpumask reservation.
> > > It tries to allocate in one call and atomically ALL the CPUs as this is the way
> > > Oprofile operates. Thus it fails if one perfmon system-wide session or one
> > > perfmon per-thread exists.
> > 
> > This only prevents oprofile from starting, but it does neither prevent
> > thread sessions nor does it prevent a perfmon per cpu session on a cpu
> > which was onlined after oprofile started, simply because it's bit is
> > missing in the CPU mask. 
> 
> The perfctr reservation is global over all CPUs.

So this mean we manage resources on multiple levels, some bits here
and some bits there and five checks in each code path do get them all.

Really convincing concept.

Thanks,

	tglx