linux-kernel - Re: [PATCH v2 02/11] perf/x86: Improve HT workaround GP counter constraint

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Message-ID: <20150527074809.GU3644@twins.programming.kicks-ass.net>
Date:	Wed, 27 May 2015 09:48:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...nel.org, vincent.weaver@...ne.edu, eranian@...gle.com,
	jolsa@...hat.com, kan.liang@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/11] perf/x86: Improve HT workaround GP counter
 constraint

On Tue, May 26, 2015 at 04:33:42PM -0700, Andi Kleen wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > +		 */
> > +		if (is_ht_workaround_enabled() && !cpuc->is_fake &&
> 
> Could this function also check if at least one leaking event is
> scheduled somewhere? (e.g. from a global count)

You truncated one line too many:

+               if (is_ht_workaround_enabled() && !cpuc->is_fake &&
+                   READ_ONCE(cpuc->excl_cntrs->exclusive_present))
+                       gpmax /= 2;

Guess what that READ_ONCE() does? It checks if there's one such leaky
event on the current core.
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