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>

1 files changed, 53 insertions, 69 deletions

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index e1d16c9a7680..ac2e1dc708bd 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -48,13 +48,6 @@ static __read_mostly int sched_clock_running;
 __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 @@ void sched_clock_init(void)
         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 @@ static inline u64 wrap_max(u64 x, u64 y)
  *  - 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 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
          */
 
         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();
-
-        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);
-        }
+        scd = cpu_sdc(cpu);
 
-        __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 @@ void sched_clock_tick(void)
         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);
 }
 
 /*