On Tue, Oct 16, 2012 at 7:23 PM, Peter Zijlstra <[email protected]> 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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
