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

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Message-Id: <20100301.222037.82133333.davem@davemloft.net>
Date:	Mon, 01 Mar 2010 22:20:37 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eranian@...gle.com
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	paulus@...ba.org, fweisbec@...il.com, perfmon2-devel@...ts.sf.net,
	robert.richter@....com, eranian@...il.com
Subject: Re: [PATCH] perf_events: add sampling period randomization support

From: eranian@...gle.com
Date: Mon,  1 Mar 2010 22:07:09 -0800

> This patch adds support for randomizing the sampling period.
> Randomization is very useful to mitigate the bias that exists
> with sampling. The random number generator does not need to
> be sophisticated. This patch uses the builtin random32()
> generator.
> 
> The user activates randomization by setting the perf_event_attr.random
> field to 1 and by passing a bitmask to control the range of variation
> above the base period. Period will vary from period to period & mask.
> Note that randomization is not available when a target interrupt rate
> (freq) is enabled.
> 
> The last used period can be collected using the PERF_SAMPLE_PERIOD flag
> in sample_type.
> 
> The patch has been tested on X86. There is also code for PowerPC but
> I could not test it.
> 
> 	Signed-off-by: Stephane Eranian <eranian@...gle.com>

Please add this, which adds the feature to sparc too.

diff --git a/arch/sparc/kernel/perf_event.c b/arch/sparc/kernel/perf_event.c
index 9f2b2ba..3e28225 100644
--- a/arch/sparc/kernel/perf_event.c
+++ b/arch/sparc/kernel/perf_event.c
@@ -1214,6 +1214,9 @@ static int __kprobes perf_event_nmi_handler(struct notifier_block *self,
 		if (val & (1ULL << 31))
 			continue;
 
+		if (event->attr.random)
+			perf_randomize_event_period(event);
+
 		data.period = event->hw.last_period;
 		if (!sparc_perf_event_set_period(event, hwc, idx))
 			continue;
--
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