Re: VolanoMark regression with 2.6.27-rc1

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2008-08-04 at 11:23 +0530, Dhaval Giani wrote:

> Peter, vatsa, any ideas?

---

Patches in tip/sched/clock

---
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5270d44..ea436bc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1572,28 +1572,13 @@ static inline void sched_clock_idle_sleep_event(void)
 static inline void sched_clock_idle_wakeup_event(u64 delta_ns)
 {
 }
-
-#ifdef CONFIG_NO_HZ
-static inline void sched_clock_tick_stop(int cpu)
-{
-}
-
-static inline void sched_clock_tick_start(int cpu)
-{
-}
-#endif
-
-#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
+#else
 extern void sched_clock_init(void);
 extern u64 sched_clock_cpu(int cpu);
 extern void sched_clock_tick(void);
 extern void sched_clock_idle_sleep_event(void);
 extern void sched_clock_idle_wakeup_event(u64 delta_ns);
-#ifdef CONFIG_NO_HZ
-extern void sched_clock_tick_stop(int cpu);
-extern void sched_clock_tick_start(int cpu);
 #endif
-#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 
 /*
  * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
diff --git a/kernel/Kconfig.hz b/kernel/Kconfig.hz
index 382dd5a..94fabd5 100644
--- a/kernel/Kconfig.hz
+++ b/kernel/Kconfig.hz
@@ -55,4 +55,4 @@ config HZ
 	default 1000 if HZ_1000
 
 config SCHED_HRTICK
-	def_bool HIGH_RES_TIMERS && USE_GENERIC_SMP_HELPERS
+	def_bool HIGH_RES_TIMERS && (!SMP || USE_GENERIC_SMP_HELPERS)
diff --git a/kernel/sched.c b/kernel/sched.c
index 21f7da9..9a76e92 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -834,7 +834,7 @@ static inline u64 global_rt_period(void)
 
 static inline u64 global_rt_runtime(void)
 {
-	if (sysctl_sched_rt_period < 0)
+	if (sysctl_sched_rt_runtime < 0)
 		return RUNTIME_INF;
 
 	return (u64)sysctl_sched_rt_runtime * NSEC_PER_USEC;
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 22ed55d..074edc9 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -32,14 +32,18 @@
 #include <linux/ktime.h>
 #include <linux/module.h>
 
+/*
+ * Scheduler clock - returns current time in nanosec units.
+ * This is default implementation.
+ * Architectures and sub-architectures can override this.
+ */
+unsigned long long __attribute__((weak)) sched_clock(void)
+{
+	return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
+}
 
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
 
-#define MULTI_SHIFT 15
-/* Max is double, Min is 1/2 */
-#define MAX_MULTI (2LL << MULTI_SHIFT)
-#define MIN_MULTI (1LL << (MULTI_SHIFT-1))
-
 struct sched_clock_data {
 	/*
 	 * Raw spinlock - this is a special case: this might be called
@@ -49,14 +53,9 @@ struct sched_clock_data {
 	raw_spinlock_t		lock;
 
 	unsigned long		tick_jiffies;
-	u64			prev_raw;
 	u64			tick_raw;
 	u64			tick_gtod;
 	u64			clock;
-	s64			multi;
-#ifdef CONFIG_NO_HZ
-	int			check_max;
-#endif
 };
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
@@ -84,90 +83,39 @@ void sched_clock_init(void)
 
 		scd->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
 		scd->tick_jiffies = now_jiffies;
-		scd->prev_raw = 0;
 		scd->tick_raw = 0;
 		scd->tick_gtod = ktime_now;
 		scd->clock = ktime_now;
-		scd->multi = 1 << MULTI_SHIFT;
-#ifdef CONFIG_NO_HZ
-		scd->check_max = 1;
-#endif
 	}
 
 	sched_clock_running = 1;
 }
 
-#ifdef CONFIG_NO_HZ
-/*
- * The dynamic ticks makes the delta jiffies inaccurate. This
- * prevents us from checking the maximum time update.
- * Disable the maximum check during stopped ticks.
- */
-void sched_clock_tick_stop(int cpu)
-{
-	struct sched_clock_data *scd = cpu_sdc(cpu);
-
-	scd->check_max = 0;
-}
-
-void sched_clock_tick_start(int cpu)
-{
-	struct sched_clock_data *scd = cpu_sdc(cpu);
-
-	scd->check_max = 1;
-}
-
-static int check_max(struct sched_clock_data *scd)
-{
-	return scd->check_max;
-}
-#else
-static int check_max(struct sched_clock_data *scd)
-{
-	return 1;
-}
-#endif /* CONFIG_NO_HZ */
-
 /*
  * update the percpu scd from the raw @now value
  *
  *  - filter out backward motion
  *  - use jiffies to generate a min,max window to clip the raw values
  */
-static void __update_sched_clock(struct sched_clock_data *scd, u64 now, u64 *time)
+static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
 {
 	unsigned long now_jiffies = jiffies;
 	long delta_jiffies = now_jiffies - scd->tick_jiffies;
 	u64 clock = scd->clock;
 	u64 min_clock, max_clock;
-	s64 delta = now - scd->prev_raw;
+	s64 delta = now - scd->tick_raw;
 
 	WARN_ON_ONCE(!irqs_disabled());
-
-	/*
-	 * At schedule tick the clock can be just under the gtod. We don't
-	 * want to push it too prematurely.
-	 */
-	min_clock = scd->tick_gtod + (delta_jiffies * TICK_NSEC);
-	if (min_clock > TICK_NSEC)
-		min_clock -= TICK_NSEC / 2;
+	min_clock = scd->tick_gtod + delta_jiffies * TICK_NSEC;
 
 	if (unlikely(delta < 0)) {
 		clock++;
 		goto out;
 	}
 
-	/*
-	 * The clock must stay within a jiffie of the gtod.
-	 * But since we may be at the start of a jiffy or the end of one
-	 * we add another jiffy buffer.
-	 */
-	max_clock = scd->tick_gtod + (2 + delta_jiffies) * TICK_NSEC;
-
-	delta *= scd->multi;
-	delta >>= MULTI_SHIFT;
+	max_clock = min_clock + TICK_NSEC;
 
-	if (unlikely(clock + delta > max_clock) && check_max(scd)) {
+	if (unlikely(clock + delta > max_clock)) {
 		if (clock < max_clock)
 			clock = max_clock;
 		else
@@ -180,12 +128,10 @@ static void __update_sched_clock(struct sched_clock_data *scd, u64 now, u64 *tim
 	if (unlikely(clock < min_clock))
 		clock = min_clock;
 
-	if (time)
-		*time = clock;
-	else {
-		scd->prev_raw = now;
-		scd->clock = clock;
-	}
+	scd->tick_jiffies = now_jiffies;
+	scd->clock = clock;
+
+	return clock;
 }
 
 static void lock_double_clock(struct sched_clock_data *data1,
@@ -203,7 +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;
@@ -212,43 +158,44 @@ 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 -= my_scd->tick_raw;
-		now += scd->tick_raw;
+		this_clock = __update_sched_clock(my_scd, now);
+		remote_clock = scd->clock;
 
-		now += my_scd->tick_gtod;
-		now -= scd->tick_gtod;
+		/*
+		 * 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);
-
-		__update_sched_clock(scd, now, &clock);
-
-		__raw_spin_unlock(&scd->lock);
-
 	} else {
 		__raw_spin_lock(&scd->lock);
-		__update_sched_clock(scd, now, NULL);
-		clock = scd->clock;
-		__raw_spin_unlock(&scd->lock);
+		clock = __update_sched_clock(scd, now);
 	}
 
+	__raw_spin_unlock(&scd->lock);
+
 	return clock;
 }
 
 void sched_clock_tick(void)
 {
 	struct sched_clock_data *scd = this_scd();
-	unsigned long now_jiffies = jiffies;
-	s64 mult, delta_gtod, delta_raw;
 	u64 now, now_gtod;
 
 	if (unlikely(!sched_clock_running))
@@ -260,29 +207,14 @@ void sched_clock_tick(void)
 	now = sched_clock();
 
 	__raw_spin_lock(&scd->lock);
-	__update_sched_clock(scd, now, NULL);
+	__update_sched_clock(scd, now);
 	/*
 	 * update tick_gtod after __update_sched_clock() because that will
 	 * already observe 1 new jiffy; adding a new tick_gtod to that would
 	 * increase the clock 2 jiffies.
 	 */
-	delta_gtod = now_gtod - scd->tick_gtod;
-	delta_raw = now - scd->tick_raw;
-
-	if ((long)delta_raw > 0) {
-		mult = delta_gtod << MULTI_SHIFT;
-		do_div(mult, delta_raw);
-		scd->multi = mult;
-		if (scd->multi > MAX_MULTI)
-			scd->multi = MAX_MULTI;
-		else if (scd->multi < MIN_MULTI)
-			scd->multi = MIN_MULTI;
-	} else
-		scd->multi = 1 << MULTI_SHIFT;
-
 	scd->tick_raw = now;
 	scd->tick_gtod = now_gtod;
-	scd->tick_jiffies = now_jiffies;
 	__raw_spin_unlock(&scd->lock);
 }
 
@@ -301,7 +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
@@ -310,9 +241,7 @@ void sched_clock_idle_wakeup_event(u64 delta_ns)
 	 * rq clock:
 	 */
 	__raw_spin_lock(&scd->lock);
-	scd->prev_raw = now;
 	scd->clock += delta_ns;
-	scd->multi = 1 << MULTI_SHIFT;
 	__raw_spin_unlock(&scd->lock);
 
 	touch_softlockup_watchdog();
@@ -321,16 +250,6 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 
 #endif
 
-/*
- * Scheduler clock - returns current time in nanosec units.
- * This is default implementation.
- * Architectures and sub-architectures can override this.
- */
-unsigned long long __attribute__((weak)) sched_clock(void)
-{
-	return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
-}
-
 unsigned long long cpu_clock(int cpu)
 {
 	unsigned long long clock;
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index cf2cd6c..0fe94ea 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -899,7 +899,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 		 * doesn't make sense. Rely on vruntime for fairness.
 		 */
 		if (rq->curr != p)
-			delta = max(10000LL, delta);
+			delta = max_t(s64, 10000LL, delta);
 
 		hrtick_start(rq, delta);
 	}
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 825b4c0..f5da526 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -289,7 +289,6 @@ void tick_nohz_stop_sched_tick(int inidle)
 			ts->tick_stopped = 1;
 			ts->idle_jiffies = last_jiffies;
 			rcu_enter_nohz();
-			sched_clock_tick_stop(cpu);
 		}
 
 		/*
@@ -392,7 +391,6 @@ void tick_nohz_restart_sched_tick(void)
 	select_nohz_load_balancer(0);
 	now = ktime_get();
 	tick_do_update_jiffies64(now);
-	sched_clock_tick_start(cpu);
 	cpu_clear(cpu, nohz_cpu_mask);
 
 	/*