Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2012-06-12 at 19:09 +0200, Stephane Eranian wrote:
> > Instead of registering a microcode notifier, why not checking the
> > availability of pebs dynamically with each syscall in
> > intel_pmu_hw_config()? It looks like intel_snb_verify_ucode() is not
> > that much expensive. We can perform the check only if the event
> could
> > be for pebs and if pebs is broken. The check could be repeated when
> > setting up a new event after ucode could potentially has been
> updated
> > (e.g. after bringing a cpu online or so).

Because you then end up with a for_each_online_cpu() loop in there,
that's not pretty and quite horrible on large systems when you need to
create nr_cpus events.

Furthermore, ucode update is the rare thing, creating events happens
much more frequently.

> That's what I had in my original version. 

Right, but you really need to check all cpus, not just the one you
happen to run on or the boot cpu.