linux-kernel - Re: [PATCH] perf_events: add sampling period randomization support

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Message-ID: <20100302141824.GG13205@erda.amd.com>
Date:	Tue, 2 Mar 2010 15:18:24 +0100
From:	Robert Richter <robert.richter@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	eranian@...gle.com, linux-kernel@...r.kernel.org, mingo@...e.hu,
	paulus@...ba.org, fweisbec@...il.com, perfmon2-devel@...ts.sf.net,
	eranian@...il.com
Subject: Re: [PATCH] perf_events: add sampling period randomization support

On 02.03.10 12:41:18, Peter Zijlstra wrote:
> On Tue, 2010-03-02 at 11:53 +0100, Robert Richter wrote:
> > 
> > Only adding the random value will lead to longer sample periods on
> > average. To compensate this you could calculate something like:
> > 
> >          event->hw.sample_period = event->attr.sample_period + (new_seed & mask) - (mask >> 1);
> 
> Or cheat and do something like:
> 
>   sample_period ^= (new_seed & mask);

This wont work, it will be asymmetric, e.g. for

 (event->attr.sample_period & mask) == 0

the offset would be always positive. Only for

 (event->attr.sample_period & mask) == (mask & ~(mask >> 1))

it is correct.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center
email: robert.richter@....com

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