Subject: Re: perf_counter: request for three more sample data options
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Paul Mackerras <paulus@samba.org>
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Date: Fri, 03 Apr 2009 09:51:17 +0200
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On Fri, 2009-04-03 at 00:25 -0700, Corey Ashford wrote:

> >> I am guessing the only difficult thing here would be obtaining the 
> >> current time from an IRQ, especially NMI handler.  Is this difficult?
> > 
> > Yes, quite :-) I'll have to see what we can do there -- we could do a
> > best effort thing with little to no guarantees I think.
> > 
> 
> Best effort would be fine, I think.  I would assume that means that 
> 99.9% of the time, you'll get a correct timestamp, and the rest are 
> rubbish?  Or would there be a way to detect when you're not able to give 
> a correct timestamp and in that case replace the timestamp field with a 
> special sentinel, like all hex f's?

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.

Also, cpu_clock() can only provide monotonicity per-cpu, if a value read
on one cpu is compared to a value read on another cpu, there can be a
drift of at most 1-2 jiffies.

Anyway, I'll prod some at this and see how much of cpu_clock() we can
get working in NMI context -- currently it just bails and returns the
last value computed.

The question to Paul is, does the powerpc sched_clock() call work in NMI
-- or hard irq disable -- context?

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