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

[Pawel Moll <pawel.moll@arm.com>]

On Thu, 2013-03-14 at 15:34 +0000, Stephane Eranian wrote:
> > Well, the timestamps themselves are already exposed to userspace
> > through the ftrace and perf data logs. All people want is to add
> > secondary data stream in the same time-line.
> >
> I agree with Peter on this. The timestamps are already visible.
> All we need is the ability to generate them for another user-level
> level data stream.

Ok, how about the code below? I must say I have some doubts about the
resolution, as there seem to be no generic way of figuring it out for
the sched_clock (the arch/arm/kernel/sched_clock.c is actually
calculating it, but than just prints it out and nothing more).

And, to summarize, we went through 3 ideas:

1. ioctl() - http://article.gmane.org/gmane.linux.kernel/1433933
2. syscall - http://article.gmane.org/gmane.linux.kernel/1437057
3. POSIX clock - below

John also suggested that maybe the perf could use CLOCK_MONOTONIC_RAW
instead of local/sched_clock().

How about a final decision?

Regards

Pawel

8<--------------------
>From c986492d38156f1fc25ab3182f0a494bb13389ce Mon Sep 17 00:00:00 2001
From: Pawel Moll <pawel.moll@arm.com>
Date: Thu, 14 Mar 2013 19:49:09 +0000
Subject: [PATCH] perf: POSIX CLOCK_PERF to report current time value

To co-relate user space events with the perf events stream
a current (as in: "what time(stamp) is it now?") time value
must be made available.

This patch adds a POSIX clock returning the perf_clock()
value and accesible from userspace:

	#include <time.h>

	struct timespec ts;

	clock_gettime(CLOCK_PERF, &ts);

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
---
 include/uapi/linux/time.h |    1 +
 kernel/events/core.c      |   20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/uapi/linux/time.h b/include/uapi/linux/time.h
index 0d3c0ed..cea16b0 100644
--- a/include/uapi/linux/time.h
+++ b/include/uapi/linux/time.h
@@ -54,6 +54,7 @@ struct itimerval {
 #define CLOCK_BOOTTIME			7
 #define CLOCK_REALTIME_ALARM		8
 #define CLOCK_BOOTTIME_ALARM		9
+#define CLOCK_PERF			10
 
 /*
  * The IDs of various hardware clocks:
diff --git a/kernel/events/core.c b/kernel/events/core.c
index b0cd865..81ca459 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -37,6 +37,7 @@
 #include <linux/ftrace_event.h>
 #include <linux/hw_breakpoint.h>
 #include <linux/mm_types.h>
+#include <linux/posix-timers.h>
 
 #include "internal.h"
 
@@ -209,6 +210,19 @@ static inline u64 perf_clock(void)
 	return local_clock();
 }
 
+static int perf_posix_clock_getres(const clockid_t which_clock,
+		struct timespec *tp)
+{
+	*tp = ns_to_timespec(TICK_NSEC);
+	return 0;
+}
+
+static int perf_posix_clock_get(clockid_t which_clock, struct timespec *tp)
+{
+	*tp = ns_to_timespec(perf_clock());
+	return 0;
+}
+
 static inline struct perf_cpu_context *
 __get_cpu_context(struct perf_event_context *ctx)
 {
@@ -7391,6 +7405,10 @@ perf_cpu_notify(struct notifier_block *self, unsigned long action, void *hcpu)
 
 void __init perf_event_init(void)
 {
+	struct k_clock perf_posix_clock = {
+		.clock_getres = perf_posix_clock_getres,
+		.clock_get = perf_posix_clock_get,
+	};
 	int ret;
 
 	idr_init(&pmu_idr);
@@ -7407,6 +7425,8 @@ void __init perf_event_init(void)
 	ret = init_hw_breakpoint();
 	WARN(ret, "hw_breakpoint initialization failed with: %d", ret);
 
+	posix_timers_register_clock(CLOCK_PERF, &perf_posix_clock);
+
 	/* do not patch jump label more than once per second */
 	jump_label_rate_limit(&perf_sched_events, HZ);
 
-- 
1.7.10.4