sched: Fix the irqtime code to deal with u64 wraps

Some ARM systems have a short sched_clock() [ which needs to be fixed
too ], but this exposed a bug in the irq_time code as well, it doesn't
deal with wraps at all.

Fix the irq_time code to deal with u64 wraps by re-writing the code to
only use delta increments, which avoids the whole issue.

Reviewed-by: Venkatesh Pallipadi <venki@google.com>
Reported-by: Mikael Pettersson <mikpe@it.uu.se>
Tested-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1292242433.6803.199.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index da14302a9857124c713bbbdf8841961c033afb94..79b557c633817aea63fe142f63e53513a5022b9e 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -636,23 +636,18 @@ static inline struct task_group *task_group(struct task_struct *p)
 
 #endif /* CONFIG_CGROUP_SCHED */
 
-static u64 irq_time_cpu(int cpu);
-static void sched_irq_time_avg_update(struct rq *rq, u64 irq_time);
+static void update_rq_clock_task(struct rq *rq, s64 delta);
 
-inline void update_rq_clock(struct rq *rq)
+static void update_rq_clock(struct rq *rq)
 {
-       int cpu = cpu_of(rq);
-       u64 irq_time;
+       s64 delta;
 
        if (rq->skip_clock_update)
                return;
 
-       rq->clock = sched_clock_cpu(cpu);
-       irq_time = irq_time_cpu(cpu);
-       if (rq->clock - irq_time > rq->clock_task)
-               rq->clock_task = rq->clock - irq_time;
-
-       sched_irq_time_avg_update(rq, irq_time);
+       delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
+       rq->clock += delta;
+       update_rq_clock_task(rq, delta);
 }
 
 /*
@@ -1946,19 +1941,20 @@ void disable_sched_clock_irqtime(void)
        sched_clock_irqtime = 0;
 }
 
-static u64 irq_time_cpu(int cpu)
+static inline u64 irq_time_cpu(int cpu)
 {
-       if (!sched_clock_irqtime)
-               return 0;
-
        return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
 }
 
+/*
+ * Called before incrementing preempt_count on {soft,}irq_enter
+ * and before decrementing preempt_count on {soft,}irq_exit.
+ */
 void account_system_vtime(struct task_struct *curr)
 {
        unsigned long flags;
+       s64 delta;
        int cpu;
-       u64 now, delta;
 
        if (!sched_clock_irqtime)
                return;
@@ -1966,9 +1962,9 @@ void account_system_vtime(struct task_struct *curr)
        local_irq_save(flags);
 
        cpu = smp_processor_id();
-       now = sched_clock_cpu(cpu);
-       delta = now - per_cpu(irq_start_time, cpu);
-       per_cpu(irq_start_time, cpu) = now;
+       delta = sched_clock_cpu(cpu) - __this_cpu_read(irq_start_time);
+       __this_cpu_add(irq_start_time, delta);
+
        /*
         * We do not account for softirq time from ksoftirqd here.
         * We want to continue accounting softirq time to ksoftirqd thread
@@ -1976,33 +1972,54 @@ void account_system_vtime(struct task_struct *curr)
         * that do not consume any time, but still wants to run.
         */
        if (hardirq_count())
-               per_cpu(cpu_hardirq_time, cpu) += delta;
+               __this_cpu_add(cpu_hardirq_time, delta);
        else if (in_serving_softirq() && !(curr->flags & PF_KSOFTIRQD))
-               per_cpu(cpu_softirq_time, cpu) += delta;
+               __this_cpu_add(cpu_softirq_time, delta);
 
        local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(account_system_vtime);
 
-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
+static void update_rq_clock_task(struct rq *rq, s64 delta)
 {
-       if (sched_clock_irqtime && sched_feat(NONIRQ_POWER)) {
-               u64 delta_irq = curr_irq_time - rq->prev_irq_time;
-               rq->prev_irq_time = curr_irq_time;
-               sched_rt_avg_update(rq, delta_irq);
-       }
+       s64 irq_delta;
+
+       irq_delta = irq_time_cpu(cpu_of(rq)) - rq->prev_irq_time;
+
+       /*
+        * Since irq_time is only updated on {soft,}irq_exit, we might run into
+        * this case when a previous update_rq_clock() happened inside a
+        * {soft,}irq region.
+        *
+        * When this happens, we stop ->clock_task and only update the
+        * prev_irq_time stamp to account for the part that fit, so that a next
+        * update will consume the rest. This ensures ->clock_task is
+        * monotonic.
+        *
+        * It does however cause some slight miss-attribution of {soft,}irq
+        * time, a more accurate solution would be to update the irq_time using
+        * the current rq->clock timestamp, except that would require using
+        * atomic ops.
+        */
+       if (irq_delta > delta)
+               irq_delta = delta;
+
+       rq->prev_irq_time += irq_delta;
+       delta -= irq_delta;
+       rq->clock_task += delta;
+
+       if (irq_delta && sched_feat(NONIRQ_POWER))
+               sched_rt_avg_update(rq, irq_delta);
 }
 
-#else
+#else /* CONFIG_IRQ_TIME_ACCOUNTING */
 
-static u64 irq_time_cpu(int cpu)
+static void update_rq_clock_task(struct rq *rq, s64 delta)
 {
-       return 0;
+       rq->clock_task += delta;
 }
 
-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time) { }
-
-#endif
+#endif /* CONFIG_IRQ_TIME_ACCOUNTING */
 
 #include "sched_idletask.c"
 #include "sched_fair.c"