[PATCH] trace: Default to using trace_global_clock is sched_clock is unstable

[Chris Wilson <chris@chris-wilson.co.uk>]

Across suspend, we may see a very large drift in timestamps if the sched
clock is unstable, prompting the global trace's ringbuffer code to warn
and suggest switching to the global clock. Preempt this request by
detecting when the sched clock is unstable (determined during
late_initcall) and automatically switching the default clock over to
trace_global_clock.

This should prevent requiring user interaction to resolve warnings such
as:

    Delta way too big! 18446743856563626466 ts=18446744054496180323 write stamp = 197932553857
    If you just came from a suspend/resume,
    please switch to the trace global clock:
    echo global > /sys/kernel/debug/tracing/trace_clock

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 kernel/trace/trace.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 13baf85b27d8..3e39aafb9bc1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -41,6 +41,7 @@
 #include <linux/nmi.h>
 #include <linux/fs.h>
 #include <linux/trace.h>
+#include <linux/sched/clock.h>
 #include <linux/sched/rt.h>
 
 #include "trace.h"
@@ -8484,6 +8485,13 @@ void __init trace_init(void)
 	trace_event_init();
 }
 
+#ifndef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+static inline bool sched_clock_stable(void)
+{
+	return true;
+}
+#endif
+
 __init static int clear_boot_tracer(void)
 {
 	/*
@@ -8500,6 +8508,10 @@ __init static int clear_boot_tracer(void)
 	       default_bootup_tracer);
 	default_bootup_tracer = NULL;
 
+	/* sched_clock_stable() is determined in late_initcall */
+	if (!trace_boot_clock && !sched_clock_stable())
+		tracing_set_clock(&global_trace, "global");
+
 	return 0;
 }
 
-- 
2.16.3

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