[PATCH 1/8] nohz: Warn if the machine can not perform nohz_full

From: Frederic Weisbecker
Date: Mon May 20 2013 - 12:04:24 EST

From: Steven Rostedt <rostedt@xxxxxxxxxxx>

If the user configures NO_HZ_FULL and defines nohz_full=XXX on the
kernel command line, or enables NO_HZ_FULL_ALL, but nohz fails
due to the machine having a unstable clock, warn about it.

We do not want users thinking that they are getting the benefit
of nohz when their machine can not support it.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
kernel/time/tick-sched.c | 5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index bc67d42..cfc798b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -178,6 +178,11 @@ static bool can_stop_full_tick(void)
if (!sched_clock_stable) {
trace_tick_stop(0, "unstable sched clock\n");
+ /*
+ * Don't allow the user to think they can get
+ * full NO_HZ with this machine.
+ */
+ WARN_ONCE(1, "NO_HZ FULL will not work with unstable sched clock");
return false;

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