Re: [RFC] perf: need to expose sched_clock to correlate usersamples with kernel samples

[Peter Zijlstra]

On Tue, 2012-10-16 at 12:13 +0200, Stephane Eranian wrote:
> Hi,
> 
> There are many situations where we want to correlate events happening at
> the user level with samples recorded in the perf_event kernel sampling buffer.
> For instance, we might want to correlate the call to a function or creation of
> a file with samples. Similarly, when we want to monitor a JVM with jitted code,
> we need to be able to correlate jitted code mappings with perf event samples
> for symbolization.
> 
> Perf_events allows timestamping of samples with PERF_SAMPLE_TIME.
> That causes each PERF_RECORD_SAMPLE to include a timestamp
> generated by calling the local_clock() -> sched_clock_cpu() function.
> 
> To make correlating user vs. kernel samples easy, we would need to
> access that sched_clock() functionality. However, none of the existing
> clock calls permit this at this point. They all return timestamps which are
> not using the same source and/or offset as sched_clock.
> 
> I believe a similar issue exists with the ftrace subsystem.
> 
> The problem needs to be adressed in a portable manner. Solutions
> based on reading TSC for the user level to reconstruct sched_clock()
> don't seem appropriate to me.
> 
> One possibility to address this limitation would be to extend clock_gettime()
> with a new clock time, e.g., CLOCK_PERF.
> 
> However, I understand that sched_clock_cpu() provides ordering guarantees only
> when invoked on the same CPU repeatedly, i.e., it's not globally synchronized.
> But we already have to deal with this problem when merging samples obtained
> from different CPU sampling buffer in per-thread mode. So this is not
> necessarily
> a showstopper.
> 
> Alternatives could be to use uprobes but that's less practical to setup.
> 
> Anyone with better ideas?

You forgot to CC the time people ;-)

I've no problem with adding CLOCK_PERF (or another/better name).

Thomas, John?

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