From: Pawel Moll <mail@xxxxxxxxxxxxx> Hello again, Back to the subject, this time with a slightly different angle... I've organised a session on the subject during the tracing minisummit at LPC 2014 in Dusseldorf. Notes taken from the discussion taken by Steven Rostedt (thanks Steve!) http://www.linuxplumbersconf.org/2014/wp-content/uploads/2014/10/LPC2014_Tracing.txt The following three patches address three main topics. They are pretty much orthogonal and (subject to small and obvious modifications) could be applied independently from each other. An executive summary of both discussion and the patches: 1. User/kernel perf timestamps correlation Thomas suggested that, instead of jumping through loops of correlation, perf should simply generate monotonic clock timestamps, instead of using sched clock. Peter and I pointed out that Ingo didn't like this idea as monotonic can be slow, but apparently the cases when it is are irrelevant. Thomas offered to fly to Budapest to physically convince Ingo - I hope it won't be necessary and he will be able to achieve this here, on mailing lists :-) Setting aside potential performance problems, it would be a really great solution, unifying all trace systems (perf, ftrace and LTTng) in this respect. I'm more than happy to work on potential improvements in the are of monotonic clock if it was to help the cause. If it is a definite no-go, we still have the third patch, allowing post- capture correlation based on synchronisation events. 2. User event generation Everyone present agreed that it would be a very-nice-to-have feature. There was some discussion about implementation details, so I welcome feedback and comments regarding my take on the matter. 3. Correlation with external timestamps This is a new issue, which surfaced recently while I was working on hardware trace infrastructure. It can timestamp trace packets, but is using yet another, completely different time source to do this. Thomas suggested solution which gets down to my original proposal for sched/monotonic clock correlation - an additional sample type so events can be "double stamped" using different clock sources providing synchronisation points for later time approximation. I've just extended the implementation with configuration value to select the clock source. If the first patch (making perf timestamps monotonic) gets accepted, there will be no immediate need for this one, but I'd like to gain some feedback anyway. That's all for this series. Previous versions: - RFC: http://www.spinics.net/lists/kernel/msg1824419.html - v1: http://thread.gmane.org/gmane.linux.kernel/1790231 - v2: http://thread.gmane.org/gmane.linux.kernel/1793272 Pawel Moll (3): perf: Use monotonic clock as a source for timestamps perf: Userspace event perf: Sample additional clock value include/linux/perf_event.h | 7 +++ include/uapi/linux/perf_event.h | 37 +++++++++++++- include/uapi/linux/prctl.h | 10 ++++ kernel/events/core.c | 105 +++++++++++++++++++++++++++++++++++++++- kernel/sys.c | 5 ++ kernel/sysctl.c | 7 +++ 6 files changed, 168 insertions(+), 3 deletions(-) -- 1.8.3.2 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
