nohz: Fix another inconsistency between CONFIG_NO_HZ=n and nohz=off

commit 0e576acbc1d9600cf2d9b4a141a2554639959d50 upstream.

If CONFIG_NO_HZ=n tick_nohz_get_sleep_length() returns NSEC_PER_SEC/HZ.

If CONFIG_NO_HZ=y and the nohz functionality is disabled via the
command line option "nohz=off" or not enabled due to missing hardware
support, then tick_nohz_get_sleep_length() returns 0. That happens
because ts->sleep_length is never set in that case.

Set it to NSEC_PER_SEC/HZ when the NOHZ mode is inactive.

Reported-by: Michal Hocko <mhocko@suse.cz>
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index e9a45f1..2695d72 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -325,8 +325,10 @@ void tick_nohz_stop_sched_tick(int inidle)
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
        }
 
-       if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
+       if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
+               ts->sleep_length = (ktime_t) { .tv64 = NSEC_PER_SEC/HZ };
                goto end;
+       }
 
        if (need_resched())
                goto end;