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SubjectRe: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
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
> 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

>> 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
>> 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.


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