Messages in this thread | | | Date | Fri, 3 Apr 2009 18:32:48 +0200 | From | Ingo Molnar <> | Subject | Re: perf_counter: request for three more sample data options |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 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.
Note that on latest mainline and on Nehalem CPUs that filter is being cut back already. So there's an opt-in mechanism to trust sched_clock() some more.
> 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.
That should be a good start i think. If it causes any measurable jitter then the performance monitoring community is probably going to be the first one to notice! ;-) So there's good synergy IMO.
Ingo
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