Re: [PATCH] perf_events: add sampling period randomization support
From: Robert Richter
Date: Tue Mar 02 2010 - 05:53:36 EST
On 01.03.10 22:07:09, eranian@xxxxxxxxxx wrote:
> 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()
> 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.
Instead of providing a mask I would prefer to either use a bit width
parameter there the mask can be calculated from or to specify a range
the period may vary.
> 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@xxxxxxxxxx>
> arch/powerpc/kernel/perf_event.c | 3 +++
> arch/x86/kernel/cpu/perf_event.c | 2 ++
> arch/x86/kernel/cpu/perf_event_intel.c | 4 ++++
I agree with Peter, I also don't see the need to touch arch specific
> include/linux/perf_event.h | 7 +++++--
> kernel/perf_event.c | 24 ++++++++++++++++++++++++
> 5 files changed, 38 insertions(+), 2 deletions(-)
> +void perf_randomize_event_period(struct perf_event *event)
> + u64 new_seed;
> + u64 mask = event->attr.random_mask;
> + event->hw.last_period = event->hw.sample_period;
> + new_seed = random32();
> + if (unlikely(mask >> 32))
> + new_seed |= (u64)random32() << 32;
> + event->hw.sample_period = event->attr.sample_period + (new_seed & mask);
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, the offset is already in sample_period.
Also a range check for sample_period is necessary to avoid over- or
Advanced Micro Devices, Inc.
Operating System Research Center
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