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

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Date:	Tue, 12 Jun 2012 19:23:34 +0200
From:	Robert Richter <robert.richter@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Stephane Eranian <eranian@...gle.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 12.06.12 19:13:23, Peter Zijlstra wrote:
> 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.

But usually the check fails on the current cpu, no need to touch other
cpus in this case. for_each_online_cpu() would run only for the case
that there just was a ucode update and pebs is going to be enabled.
And for this rare case we could use locking.

> 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.

Once pebs is enabled no further checks are needed anymore, I think.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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