nohz: Prevent clocksource wrapping during idle

The dynamic tick allows the kernel to sleep for periods longer than a
single tick, but it does not limit the sleep time currently. In the
worst case the kernel could sleep longer than the wrap around time of
the time keeping clock source which would result in losing track of
time.

Prevent this by limiting it to the safe maximum sleep time of the
current time keeping clock source. The value is calculated when the
clock source is registered.

[ tglx: simplified the code a bit and massaged the commit msg ]

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Cc: John Stultz <johnstul@us.ibm.com>
LKML-Reference: <1250617512-23567-2-git-send-email-jon-hunter@ti.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index f57f88250526..279c5478e8a6 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -151,6 +151,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
  *                      subtraction of non 64 bit counters
  * @mult:               cycle to nanosecond multiplier
  * @shift:              cycle to nanosecond divisor (power of two)
+ * @max_idle_ns:        max idle time permitted by the clocksource (nsecs)
  * @flags:              flags describing special properties
  * @vread:              vsyscall based read
  * @resume:             resume function for the clocksource, if necessary
@@ -168,6 +169,7 @@ struct clocksource {
         cycle_t mask;
         u32 mult;
         u32 shift;
+        u64 max_idle_ns;
         unsigned long flags;
         cycle_t (*vread)(void);
         void (*resume)(void);
diff --git a/include/linux/time.h b/include/linux/time.h
index fe04e5ef6a59..6e026e45a179 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -148,6 +148,7 @@ extern void monotonic_to_bootbased(struct timespec *ts);
 
 extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
 extern int timekeeping_valid_for_hres(void);
+extern u64 timekeeping_max_deferment(void);
 extern void update_wall_time(void);
 extern void update_xtime_cache(u64 nsec);
 extern void timekeeping_leap_insert(int leapsecond);