Messages in this thread | | | Date | Fri, 22 Feb 2013 22:04:33 -0800 | From | John Stultz <> | Subject | Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples |
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On 02/20/2013 02:29 AM, Peter Zijlstra wrote: > On Tue, 2013-02-19 at 10:25 -0800, John Stultz wrote: >> So describe how the perf time domain is different then >> CLOCK_MONOTONIC_RAW. > The primary difference is that the trace/sched/perf time domain is not > strictly monotonic, it is only locally monotonic -- that is two time > stamps taken on the same cpu are guaranteed to be monotonic.
So how would a clock_gettime(CLOCK_PERF,...) interface help you figure out which cpu you got your timestamp from?
> Furthermore, to make it useful, there's an actual bound on the inter-cpu > drift (implemented by limiting the drift to CLOCK_MONOTONIC).
So this sounds like you're already sort of interpolating to CLOCK_MONOTONIC, or am I just misunderstanding you?
> Additionally -- to increase use -- we also added a monotonic sync point > when cpu A queries time of cpu B.
Not sure I'm following this bit. But I'll have to go look at the code on Monday.
> >> My concern here is that we're basically creating a kernel interface >> that >> exports implementation-defined semantics (again: whatever perf does >> right now). And I think folks want to do this, because adding >> CLOCK_PERF >> is easier then trying to: >> >> 1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW >> >> 2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or >> CLOCKMONOTONIC_RAW when it exports the data > Mostly cheaper, not easier. Given unstable TSC, MONOTONIC will have to > fall back to another clock source (hpet, acpi_pm and other assorted > crap). > > In order to avoid this, we'd had to relax the requirements. Using > anything other than TSC is simply not an option.
Right, and this I understand. We can can play a little fast and lose with the rules for in-kernel uses, given the variety of hardware and the fact that performance is more critical then perfect accuracy. Since we're in-kernel we also have more information then userland does about what cpu we're running on, so we can get away with only locally-monotonic timestamps.
But I want to be careful if we're exporting this out to userland that its both useful and that there's an actual specification for how CLOCK_PERF behaves, applications can rely upon not changing in the future.
thanks -john
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