[S390] lockless idle time accounting

Replace the spinlock used in the idle time accounting with a sequence
counter mechanism analog to seqlock.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

3 files changed, 35 insertions, 17 deletions

diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h
index ec917d42ee6d..7a3817a656df 100644
--- a/arch/s390/include/asm/cputime.h
+++ b/arch/s390/include/asm/cputime.h
@@ -178,7 +178,7 @@ cputime64_to_clock_t(cputime64_t cputime)
 }
 
 struct s390_idle_data {
-        spinlock_t lock;
+        unsigned int sequence;
         unsigned long long idle_count;
         unsigned long long idle_enter;
         unsigned long long idle_time;
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index fd8e3111a4e8..2270730f5354 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -856,13 +856,20 @@ static ssize_t show_idle_count(struct sys_device *dev,
 {
         struct s390_idle_data *idle;
         unsigned long long idle_count;
+        unsigned int sequence;
 
         idle = &per_cpu(s390_idle, dev->id);
-        spin_lock(&idle->lock);
+repeat:
+        sequence = idle->sequence;
+        smp_rmb();
+        if (sequence & 1)
+                goto repeat;
         idle_count = idle->idle_count;
         if (idle->idle_enter)
                 idle_count++;
-        spin_unlock(&idle->lock);
+        smp_rmb();
+        if (idle->sequence != sequence)
+                goto repeat;
         return sprintf(buf, "%llu\n", idle_count);
 }
 static SYSDEV_ATTR(idle_count, 0444, show_idle_count, NULL);
@@ -872,15 +879,22 @@ static ssize_t show_idle_time(struct sys_device *dev,
 {
         struct s390_idle_data *idle;
         unsigned long long now, idle_time, idle_enter;
+        unsigned int sequence;
 
         idle = &per_cpu(s390_idle, dev->id);
-        spin_lock(&idle->lock);
         now = get_clock();
+repeat:
+        sequence = idle->sequence;
+        smp_rmb();
+        if (sequence & 1)
+                goto repeat;
         idle_time = idle->idle_time;
         idle_enter = idle->idle_enter;
         if (idle_enter != 0ULL && idle_enter < now)
                 idle_time += now - idle_enter;
-        spin_unlock(&idle->lock);
+        smp_rmb();
+        if (idle->sequence != sequence)
+                goto repeat;
         return sprintf(buf, "%llu\n", idle_time >> 12);
 }
 static SYSDEV_ATTR(idle_time_us, 0444, show_idle_time, NULL);
@@ -908,11 +922,7 @@ static int __cpuinit smp_cpu_notify(struct notifier_block *self,
         case CPU_ONLINE:
         case CPU_ONLINE_FROZEN:
                 idle = &per_cpu(s390_idle, cpu);
-                spin_lock_irq(&idle->lock);
-                idle->idle_enter = 0;
-                idle->idle_time = 0;
-                idle->idle_count = 0;
-                spin_unlock_irq(&idle->lock);
+                memset(idle, 0, sizeof(struct s390_idle_data));
                 if (sysfs_create_group(&s->kobj, &cpu_online_attr_group))
                         return NOTIFY_BAD;
                 break;
diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c
index ade17e771f05..c41bb0d416e1 100644
--- a/arch/s390/kernel/vtime.c
+++ b/arch/s390/kernel/vtime.c
@@ -27,9 +27,7 @@
 
 static DEFINE_PER_CPU(struct vtimer_queue, virt_cpu_timer);
 
-DEFINE_PER_CPU(struct s390_idle_data, s390_idle) = {
-        .lock = __SPIN_LOCK_UNLOCKED(s390_idle.lock)
-};
+DEFINE_PER_CPU(struct s390_idle_data, s390_idle);
 
 static inline __u64 get_vtimer(void)
 {
@@ -151,11 +149,13 @@ void vtime_start_cpu(void)
                 vq->elapsed -= vq->idle - S390_lowcore.async_enter_timer;
         }
 
-        spin_lock(&idle->lock);
+        idle->sequence++;
+        smp_wmb();
         idle->idle_time += idle_time;
         idle->idle_enter = 0ULL;
         idle->idle_count++;
-        spin_unlock(&idle->lock);
+        smp_wmb();
+        idle->sequence++;
 }
 
 void vtime_stop_cpu(void)
@@ -242,15 +242,23 @@ cputime64_t s390_get_idle_time(int cpu)
 {
         struct s390_idle_data *idle;
         unsigned long long now, idle_time, idle_enter;
+        unsigned int sequence;
 
         idle = &per_cpu(s390_idle, cpu);
-        spin_lock(&idle->lock);
+
         now = get_clock();
+repeat:
+        sequence = idle->sequence;
+        smp_rmb();
+        if (sequence & 1)
+                goto repeat;
         idle_time = 0;
         idle_enter = idle->idle_enter;
         if (idle_enter != 0ULL && idle_enter < now)
                 idle_time = now - idle_enter;
-        spin_unlock(&idle->lock);
+        smp_rmb();
+        if (idle->sequence != sequence)
+                goto repeat;
         return idle_time;
 }