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Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: "Metzger, Markus T" <markus.t.metzger@intel.com>
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Date: Fri, 07 Aug 2009 15:05:58 +0200
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On Fri, 2009-08-07 at 13:18 +0100, Metzger, Markus T wrote:

> >Right, what I'm worried about though is the BTS overload scenario.
> >Normally when we'd create more counters than we'd have hardware for we'd
> >simply time share the stuff.
> >
> >However BTS now has a second class fallback for period==1 which
> >complicates all this because it will likely not generate consistent
> >results.
> >
> >So I was thinking that _if_ the hardware supports BTS we'd not do the
> >fallback to generic bits if event == HW_BRANCH_INST && period == 1.
> >
> >I agree on the period > 1 using the generic counters.
> 
> 
> OK, that makes sense.
> 
> So I'll still check for sample_period=1 but when I fail to acquire BTS, I don't
> fall back to the generic counter and return an error instead.
> 
> Is that it?

Yeah, something like that, simply return -EBUSY/-EAGAIN or so.

However I'd look into adding something to intel_pmu_init() which would
set intel_bts_available to 1 when the CPU should support BTS, that'll
ease the reserve_bts_hardware() error case and help with allowing this
fallback as well.