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From: "Robert O'Callahan" <robert@ocallahan.org>
Date: Wed, 28 Jun 2017 10:48:33 -0700
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Subject: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the
 kernel in the "skid" region
To: Mark Rutland <mark.rutland@arm.com>
Cc: Kyle Huey <me@kylehuey.com>, "Jin, Yao" <yao.jin@linux.intel.com>,
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On Wed, Jun 28, 2017 at 10:19 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> It seems odd that an event without any samples to take has a sample
> period. I'm surprised that there's not *some* sample_type set.

Yes, it is a bit odd :-). Rather than trying to gather some limited
set of information about the target process, we want to interrupt its
execution after a particular number of events have occurred.

The bigger picture here is that we're replaying a previously recorded
process execution. We want to the process to run until it reaches a
particular state that occurred during recording; that state is
identified by a value for the performance counter (retired conditional
branches) plus the values of the general-purpose registers. So our
first step is to run the process until the performance counter reaches
a value less than but "close to" the target value. (We know the
process will be interrupted after a number of extra events have
occurred, so we set the sample period to a value less than the target
by a heuristic "maximum skid size".)

Rob
-- 
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