linux-kernel - Re: perf_counter: request for three more sample data options

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Message-ID: <20090403163756.GH3226@erda.amd.com>
Date:	Fri, 3 Apr 2009 18:38:01 +0200
From:	Robert Richter <robert.richter@....com>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: perf_counter: request for three more sample data options

On 03.04.09 19:51:11, Paul Mackerras wrote:
> Peter Zijlstra writes:
> 
> > What I was thinking of was re-using some of the cpu_clock()
> > infrastructure. That provides us with a jiffy based GTOD sample,
> > cpu_clock() then uses TSC and a few filters to compute a current
> > timestamp.
> > 
> > I was thinking about cutting back those filters and thus trusting the
> > TSC more -- which on x86 can do any random odd thing. So provided the
> > TSC is not doing funny the results will be ok-ish.
> > 
> > This does mean however, that its not possible to know when its gone bad.
> 
> I would expect that perfmon would be just reading the TSC and
> recording that.  If you can read the TSC and do some correction then
> we're ahead. :)
> 
> > The question to Paul is, does the powerpc sched_clock() call work in NMI
> > -- or hard irq disable -- context?
> 
> Yes - timekeeping is one area where us powerpc guys can be smug. :)
> We have a per-core, 64-bit timebase register which counts at a
> constant frequency and is synchronized across all cores.  So
> sched_clock works in any context on powerpc - all it does is read the
> timebase and do some simple integer arithmetic on it.

Ftrace is using ring_buffer_time_stamp() that finally uses
sched_clock(). But I am not sure if the time is correct when calling
from an NMI handler.

-Robert

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

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