linux-kernel - Re: [PATCH 3/4] perf, x86: Use ALTERNATIVE() to check for X86_FEATURE_PERFCTR

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Message-ID: <20110419182148.GS16484@one.firstfloor.org>
Date:	Tue, 19 Apr 2011 20:21:48 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Robert Richter <robert.richter@....com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] perf, x86: Use ALTERNATIVE() to check for X86_FEATURE_PERFCTR_CORE

On Tue, Apr 19, 2011 at 12:39:27PM +0200, Robert Richter wrote:
> On 18.04.11 16:00:57, Andi Kleen wrote:
> > Robert Richter <robert.richter@....com> writes:
> > 
> > > Using ALTERNATIVE() when checking for X86_FEATURE_PERFCTR_CORE avoids
> > > an extra pointer chase and data cache hit.
> > 
> > Is that really a performance critical path?
> > 
> > Seems more like unnecessary obfuscation to me.
> 
> We hotest path is in perf_pmu_disable(), which happens at least with
> every task switch when calling the event scheduler.

Yes but that's already a slow path isn't it? It better is, because
the MSR accesses alone are incredibly expensive. I guess your test
and jump isn't even on the radar after that ...

-Andi
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