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

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Date:	Tue, 12 Jun 2012 19:13:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Robert Richter <robert.richter@....com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, mingo@...e.hu, ming.m.lin@...el.com,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Borislav Petkov <borislav.petkov@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on
 SandyBridge

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.
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