linux-kernel - Re: [PATCH/RFC] perfcounters: record time running and time enabled for each counter

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Message-ID: <49C3F8FE.4050406@linux.vnet.ibm.com>
Date:	Fri, 20 Mar 2009 13:13:50 -0700
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] perfcounters: record time running and time   enabled
 for each counter

Paul Mackerras wrote:
> Impact: new functionality
> 
> Currently, if there are more counters enabled than can fit on the CPU,
> the kernel will multiplex the counters on to the hardware using
> round-robin scheduling.  That isn't too bad for sampling counters, but
> for counting counters it means that the value read from a counter
> represents some unknown fraction of the true count of events that
> occurred while the counter was enabled.
> 
> This remedies the situation by keeping track of how long each counter
> is enabled for, and how long it is actually on the cpu and counting
> events.  These times are recorded in nanoseconds using the task clock
> for per-task counters and the cpu clock for per-cpu counters.
> 
> These values can be supplied to userspace on a read from the counter.
> Userspace requests that they be supplied after the counter value by
> setting the PERF_FORMAT_TIME_ENABLED and/or PERF_FORMAT_TIME_RUNNING
> bits in the hw_event.read_format field when creating the counter.
> (There is no way to change the read format after the counter is
> created, though it would be possible to add some way to do that.)
[snip]

This would make the implementation of PAPI simpler than using software 
CPU timers attached to each group (which I have prototyped).

I'm a little confused about the ABI, though.  I see that I can specify 
which times I want to be able to read, but the order that they appear in 
the stream seems to be unstated.  I tried to figure it out by looking at 
  the kernel code, but there is no reference to PERF_FORMAT_TIME_ENABLED 
or PERF_FORMAT_TIME_RUNNING in the patch (besides the definition).


Regards,

- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR
503-578-3507
cjashfor@...ibm.com

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