sched clock: couple local and remote clocks

When taking the time of a remote CPU, use the opportunity to
couple (sync) the clocks to each other. (in a monotonic way)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>

1 files changed, 21 insertions, 12 deletions

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 857a1291fd23..074edc989379 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -149,7 +149,7 @@ static void lock_double_clock(struct sched_clock_data *data1,
 u64 sched_clock_cpu(int cpu)
 {
         struct sched_clock_data *scd = cpu_sdc(cpu);
-        u64 now, clock;
+        u64 now, clock, this_clock, remote_clock;
 
         if (unlikely(!sched_clock_running))
                 return 0ull;
@@ -158,26 +158,36 @@ u64 sched_clock_cpu(int cpu)
         now = sched_clock();
 
         if (cpu != raw_smp_processor_id()) {
-                /*
-                 * in order to update a remote cpu's clock based on our
-                 * unstable raw time rebase it against:
-                 *   tick_raw           (offset between raw counters)
-                 *   tick_gotd          (tick offset between cpus)
-                 */
                 struct sched_clock_data *my_scd = this_scd();
 
                 lock_double_clock(scd, my_scd);
 
-                now += scd->tick_raw - my_scd->tick_raw;
-                now += my_scd->tick_gtod - scd->tick_gtod;
+                this_clock = __update_sched_clock(my_scd, now);
+                remote_clock = scd->clock;
+
+                /*
+                 * Use the opportunity that we have both locks
+                 * taken to couple the two clocks: we take the
+                 * larger time as the latest time for both
+                 * runqueues. (this creates monotonic movement)
+                 */
+                if (likely(remote_clock < this_clock)) {
+                        clock = this_clock;
+                        scd->clock = clock;
+                } else {
+                        /*
+                         * Should be rare, but possible:
+                         */
+                        clock = remote_clock;
+                        my_scd->clock = remote_clock;
+                }
 
                 __raw_spin_unlock(&my_scd->lock);
         } else {
                 __raw_spin_lock(&scd->lock);
+                clock = __update_sched_clock(scd, now);
         }
 
-        clock = __update_sched_clock(scd, now);
-
         __raw_spin_unlock(&scd->lock);
 
         return clock;
@@ -223,7 +233,6 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
 void sched_clock_idle_wakeup_event(u64 delta_ns)
 {
         struct sched_clock_data *scd = this_scd();
-        u64 now = sched_clock();
 
         /*
          * Override the previous timestamp and ignore all