Re: [RFC] perf: need to expose sched_clock to correlate user sampleswith kernel samples

[Stephane Eranian]

On Tue, Oct 16, 2012 at 7:23 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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 did not know where they were.

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

> Thomas, John?
>
Any comment?
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