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>
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 857a129..074edc9 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -149,7 +149,7 @@
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 @@
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 @@
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