linux-kernel - Re: [RFC PATCH 5/7] perf, x86: use the PEBS auto reload mechanism when possible

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Date:	Wed, 28 May 2014 07:46:43 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, eranian@...gle.com
Subject: Re: [RFC PATCH 5/7] perf, x86: use the PEBS auto reload mechanism
 when possible

On Wed, May 28, 2014 at 09:59:37AM +0200, Peter Zijlstra wrote:
> On Wed, May 28, 2014 at 02:18:08PM +0800, Yan, Zheng wrote:
> > When a fixed period is specified, this patch make perf use the PEBS
> > auto reload mechanism. This makes normal profiling faster, because
> > it avoids one costly MSR write in the PMI handler.
> > 
> 
> You can't do this unconditionally. Like I think you mention in the next
> patch, we need the PMI to fix up lots of cases.

There are two cases:

(1) Fixed period specified with PEBS.
The PMI still runs with threshold=1, but we can use the PEBS
auto-reload mechanism to automatically rewrite the counter which is
faster.

(2) Free running mode with more constraints

This is case (1)

-andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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