Re: [RFC] perf: need to expose sched_clock to correlate user sampleswith kernel samples
From: John Stultz
Date: Sat Feb 23 2013 - 01:04:52 EST
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 thenThe 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
My concern here is that we're basically creating a kernel interfaceMostly cheaper, not easier. Given unstable TSC, MONOTONIC will have to
exports implementation-defined semantics (again: whatever perf does
right now). And I think folks want to do this, because adding
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
fall back to another clock source (hpet, acpi_pm and other assorted
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
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.
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