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

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Message-ID: <18901.52735.579687.568717@cargo.ozlabs.ibm.com>
Date:	Fri, 3 Apr 2009 19:51:11 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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

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.

Paul.
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