nohz: Do not warn about unstable tsc unless user uses nohz_full

If the user enables CONFIG_NO_HZ_FULL and runs the kernel on a machine
with an unstable TSC, it will produce a WARN_ON dump as well as taint
the kernel. This is a bit extreme for a kernel that just enables a
feature but doesn't use it.

The warning should only happen if the user tries to use the feature by
either adding nohz_full to the kernel command line, or by enabling
CONFIG_NO_HZ_FULL_ALL that makes nohz used on all CPUs at boot up. Note,
this second feature should not (yet) be used by distros or anyone that
doesn't care if NO_HZ is used or not.

Signed-off-by: Steven Rostedt <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Li Zhong <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Kevin Hilman <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index e80183f4a6c4f3d6abea4b6d7720e381381259e0..1102534a1a575919a2aca4e87630fedc72ca4609 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -182,7 +182,8 @@ static bool can_stop_full_tick(void)
                 * 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");
+               WARN_ONCE(have_nohz_full_mask,
+                         "NO_HZ FULL will not work with unstable sched clock");
                return false;
        }
 #endif