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

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Date:	Fri, 08 Jun 2012 20:52:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <borislav.petkov@....com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, andi@...stfloor.org, mingo@...e.hu,
	ming.m.lin@...el.com, Andreas Herrmann <andreas.herrmann3@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on
 SandyBridge

On Fri, 2012-06-08 at 20:05 +0200, Borislav Petkov wrote:
> How about this: since the ucode cannot be downgraded and since higher
> ucode versions are supposed to fix current and older problems (otherwise
> ucoders will get an earlfull) you shouldn't be needing to verify the
> ucode version on all CPUs per-CPU, i.e. the O(n^2) overhead.
> 
> Rather, simply track which CPUs _haven't_ been updated yet, and once
> this is the empty set, run the verify thing to check ucode version on
> all CPUs.
> 
> And this should happen only when we update ucode from version A to
> version B, where B > A.
> 
> And unless I'm missing something, this should be O(n) and ucode update
> should happen very seldomly anyway. 

Checking a bitmap of n bits for being all zero is O(n), so the total is
still O(n^2). Still, probably faster than the for_each_online_cpu() scan
I do now.


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