[tip:perf/core] tracing: Explain about unstable clock on resume with ring buffer warning

[tip-bot for Jiri Olsa <jolsa@redhat.com>]

Commit-ID:  31274d72f01604f4b02d933b4f3cac84d2c201fd
Gitweb:     http://git.kernel.org/tip/31274d72f01604f4b02d933b4f3cac84d2c201fd
Author:     Jiri Olsa <jolsa@redhat.com>
AuthorDate: Fri, 18 Feb 2011 15:52:19 +0100
Committer:  Steven Rostedt <rostedt@goodmis.org>
CommitDate: Thu, 10 Mar 2011 10:34:47 -0500

tracing: Explain about unstable clock on resume with ring buffer warning

The "Delta way too big" warning might appear on a system with a
unstable shed clock right after the system is resumed and tracing
was enabled at time of suspend.

Since it's not realy a bug, and the unstable sched clock is working
fast and reliable otherwise, Steven suggested to keep using the
sched clock in any case and just to make note in the warning itself.

v2 changes:
- added #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
LKML-Reference: <20110218145219.GD2604@jolsa.brq.redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/ring_buffer.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 3237d96..db7b439 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2172,11 +2172,19 @@ rb_reserve_next_event(struct ring_buffer *buffer,
 	if (likely(ts >= cpu_buffer->write_stamp)) {
 		delta = diff;
 		if (unlikely(test_time_stamp(delta))) {
+			int local_clock_stable = 1;
+#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+			local_clock_stable = sched_clock_stable;
+#endif
 			WARN_ONCE(delta > (1ULL << 59),
-				  KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n",
+				  KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
 				  (unsigned long long)delta,
 				  (unsigned long long)ts,
-				  (unsigned long long)cpu_buffer->write_stamp);
+				  (unsigned long long)cpu_buffer->write_stamp,
+				  local_clock_stable ? "" :
+				  "If you just came from a suspend/resume,\n"
+				  "please switch to the trace global clock:\n"
+				  "  echo global > /sys/kernel/debug/tracing/trace_clock\n");
 			add_timestamp = 1;
 		}
 	}