nohz: Add basic tracing

It's not obvious to find out why the full dynticks subsystem
doesn't always stop the tick: whether this is due to kthreads,
posix timers, perf events, etc...

These new tracepoints are here to help the user diagnose
the failures and test this feature.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 15 insertions, 4 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 12a900dbb819..85e05ab98253 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -28,6 +28,8 @@
 
 #include "tick-internal.h"
 
+#include <trace/events/timer.h>
+
 /*
  * Per cpu nohz control structure
  */
@@ -153,14 +155,20 @@ static bool can_stop_full_tick(void)
 {
         WARN_ON_ONCE(!irqs_disabled());
 
-        if (!sched_can_stop_tick())
+        if (!sched_can_stop_tick()) {
+                trace_tick_stop(0, "more than 1 task in runqueue\n");
                 return false;
+        }
 
-        if (!posix_cpu_timers_can_stop_tick(current))
+        if (!posix_cpu_timers_can_stop_tick(current)) {
+                trace_tick_stop(0, "posix timers running\n");
                 return false;
+        }
 
-        if (!perf_event_can_stop_tick())
+        if (!perf_event_can_stop_tick()) {
+                trace_tick_stop(0, "perf events running\n");
                 return false;
+        }
 
         /* sched_clock_tick() needs us? */
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
@@ -168,8 +176,10 @@ static bool can_stop_full_tick(void)
          * TODO: kick full dynticks CPUs when
          * sched_clock_stable is set.
          */
-        if (!sched_clock_stable)
+        if (!sched_clock_stable) {
+                trace_tick_stop(0, "unstable sched clock\n");
                 return false;
+        }
 #endif
 
         return true;
@@ -631,6 +641,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 
                         ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
                         ts->tick_stopped = 1;
+                        trace_tick_stop(1, " ");
                 }
 
                 /*