Messages in this thread | | | Date | Thu, 14 Mar 2013 16:34:02 +0100 | Subject | Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples | From | Stephane Eranian <> |
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On Mon, Feb 25, 2013 at 3:10 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Fri, 2013-02-22 at 22:04 -0800, John Stultz wrote: >> 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? > > I'm not sure we want to expose it that far.. The reason people want > this clock exposed is to be able to do logging on the same time-line so > we can correlate events from both sources (kernel and user-space). > > In case of parallel execution we cannot guarantee order and reading > logs/reconstructing events things require a bit of human intelligence. > >> > 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? > > That's right, although there's modes where the TSC is guaranteed stable > where we don't do this (it avoids some expensive bits), so we can not > rely on this. > >> > 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. > > It will basically pull the 'slowest' cpu forward so that for that > 'event' we can say the two time-lines have a common point. > >> 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. > > Well, the timestamps themselves are already exposed to userspace > through the ftrace and perf data logs. All people want is to add > secondary data stream in the same time-line. > I agree with Peter on this. The timestamps are already visible. All we need is the ability to generate them for another user-level level data stream.
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