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

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Message-ID: <20090403170545.GE19982@elte.hu>
Date:	Fri, 3 Apr 2009 19:05:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Robert Richter <robert.richter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: perf_counter: request for three more sample data options


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Fri, 2009-04-03 at 18:41 +0200, Ingo Molnar wrote:
> > * Robert Richter <robert.richter@....com> wrote:
> > 
> > > 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.
> > 
> > Yeah, that's a bit icky. Right now we have the following 
> > accelerator:
> > 
> > u64 sched_clock_cpu(int cpu)
> > {
> >         u64 now, clock, this_clock, remote_clock;
> >         struct sched_clock_data *scd;
> > 
> >         if (sched_clock_stable)
> >                 return sched_clock();
> > 
> > which works rather well on CPUs that set sched_clock_stable. Do you 
> > think we could set it on Barcelona?
> 
> I think you should couple it to the tsc clocksource detection 
> thingy. On all systems the tsc is good enough to use as 
> clocksource, we can short-circuit.

No principal objections, if it works.

	Ingo
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