sched_clock: Make it NMI safe
Arjan complained about the suckyness of TSC on modern machines, and
asked if we could do something about that for PERF_SAMPLE_TIME.
Make cpu_clock() NMI safe by removing the spinlock and using
cmpxchg. This also makes it smaller and more robust.
Affects architectures that use HAVE_UNSTABLE_SCHED_CLOCK, i.e. IA64
and x86.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index e1d16c9..ac2e1dc 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -48,13 +48,6 @@
__read_mostly int sched_clock_stable;
struct sched_clock_data {
- /*
- * Raw spinlock - this is a special case: this might be called
- * from within instrumentation code so we dont want to do any
- * instrumentation ourselves.
- */
- raw_spinlock_t lock;
-
u64 tick_raw;
u64 tick_gtod;
u64 clock;
@@ -80,7 +73,6 @@
for_each_possible_cpu(cpu) {
struct sched_clock_data *scd = cpu_sdc(cpu);
- scd->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
scd->tick_raw = 0;
scd->tick_gtod = ktime_now;
scd->clock = ktime_now;
@@ -109,14 +101,19 @@
* - filter out backward motion
* - use the GTOD tick value to create a window to filter crazy TSC values
*/
-static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
+static u64 sched_clock_local(struct sched_clock_data *scd)
{
- s64 delta = now - scd->tick_raw;
- u64 clock, min_clock, max_clock;
+ u64 now, clock, old_clock, min_clock, max_clock;
+ s64 delta;
+again:
+ now = sched_clock();
+ delta = now - scd->tick_raw;
if (unlikely(delta < 0))
delta = 0;
+ old_clock = scd->clock;
+
/*
* scd->clock = clamp(scd->tick_gtod + delta,
* max(scd->tick_gtod, scd->clock),
@@ -124,84 +121,73 @@
*/
clock = scd->tick_gtod + delta;
- min_clock = wrap_max(scd->tick_gtod, scd->clock);
- max_clock = wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC);
+ min_clock = wrap_max(scd->tick_gtod, old_clock);
+ max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
clock = wrap_max(clock, min_clock);
clock = wrap_min(clock, max_clock);
- scd->clock = clock;
+ if (cmpxchg(&scd->clock, old_clock, clock) != old_clock)
+ goto again;
- return scd->clock;
+ return clock;
}
-static void lock_double_clock(struct sched_clock_data *data1,
- struct sched_clock_data *data2)
+static u64 sched_clock_remote(struct sched_clock_data *scd)
{
- if (data1 < data2) {
- __raw_spin_lock(&data1->lock);
- __raw_spin_lock(&data2->lock);
+ struct sched_clock_data *my_scd = this_scd();
+ u64 this_clock, remote_clock;
+ u64 *ptr, old_val, val;
+
+ sched_clock_local(my_scd);
+again:
+ this_clock = my_scd->clock;
+ 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((s64)(remote_clock - this_clock) < 0)) {
+ ptr = &scd->clock;
+ old_val = remote_clock;
+ val = this_clock;
} else {
- __raw_spin_lock(&data2->lock);
- __raw_spin_lock(&data1->lock);
+ /*
+ * Should be rare, but possible:
+ */
+ ptr = &my_scd->clock;
+ old_val = this_clock;
+ val = remote_clock;
}
+
+ if (cmpxchg(ptr, old_val, val) != old_val)
+ goto again;
+
+ return val;
}
u64 sched_clock_cpu(int cpu)
{
- u64 now, clock, this_clock, remote_clock;
struct sched_clock_data *scd;
+ u64 clock;
+
+ WARN_ON_ONCE(!irqs_disabled());
if (sched_clock_stable)
return sched_clock();
- scd = cpu_sdc(cpu);
-
- /*
- * Normally this is not called in NMI context - but if it is,
- * trying to do any locking here is totally lethal.
- */
- if (unlikely(in_nmi()))
- return scd->clock;
-
if (unlikely(!sched_clock_running))
return 0ull;
- WARN_ON_ONCE(!irqs_disabled());
- now = sched_clock();
+ scd = cpu_sdc(cpu);
- if (cpu != raw_smp_processor_id()) {
- struct sched_clock_data *my_scd = this_scd();
-
- lock_double_clock(scd, my_scd);
-
- 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((s64)(remote_clock - this_clock) < 0)) {
- 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);
- }
-
- __raw_spin_unlock(&scd->lock);
+ if (cpu != smp_processor_id())
+ clock = sched_clock_remote(scd);
+ else
+ clock = sched_clock_local(scd);
return clock;
}
@@ -223,11 +209,9 @@
now_gtod = ktime_to_ns(ktime_get());
now = sched_clock();
- __raw_spin_lock(&scd->lock);
scd->tick_raw = now;
scd->tick_gtod = now_gtod;
- __update_sched_clock(scd, now);
- __raw_spin_unlock(&scd->lock);
+ sched_clock_local(scd);
}
/*
