Re: [PATCH 1/2] perf: Add sampling of the raw monotonic clock

On Mon, 2014-09-29 at 16:28 +0100, Peter Zijlstra wrote:

On Thu, Sep 18, 2014 at 03:34:32PM +0100, Pawel Moll wrote:

quoted

@@ -4456,6 +4459,13 @@ static void __perf_event_header__init_id(struct perf_event_header *header,
 		data->cpu_entry.cpu	 = raw_smp_processor_id();
 		data->cpu_entry.reserved = 0;
 	}
+
+	if (sample_type & PERF_SAMPLE_CLOCK_RAW_MONOTONIC) {
+		struct timespec now;
+
+		getrawmonotonic(&now);
+		data->clock_raw_monotonic = timespec_to_ns(&now);
+	}
 }
 

This cannot work, getrawmonotonic() isn't NMI-safe and there's
nothing stopping this being used from NMI context.

Also getrawmonotonic() + timespec_to_ns() will make tglx sad, he's just
done a tree-wide eradication of silly conversions and now you're adding
a ns -> timespec -> ns dance right back.

Last thing I want is to make Thomas sad... For obvious reasons ;-)

I _think_ you want ktime_get_mono_fast_ns(), 

With pleasure, it's exactly what I need.

but this does bring us
right back to the question/discussion on which timebase you'd want to
sync again. MONO does make sense for most cases, but I think we've had
fairly sane stories for people wanting to sync against other clocks.

Yes. I've asked the same question somewhere in the thread.

ftrace has got a switch and a selection of trace_clocks in
kernel/trace/trace.c - do we want something similar (in integer form
probably, though) in perf_events.h with an additional "flag" in struct
perf_event_attr? It could be used to pick a time source for
PERF_SAMPLE_CLOCK (PERF_SAMPLE_TRACE_CLOCK?) sample.

Pawel