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Subject[tip:sched/urgent] sched: Fix the irqtime code to deal with u64 wraps
Commit-ID:  fe44d62122829959e960bc699318d58966922a69
Gitweb: http://git.kernel.org/tip/fe44d62122829959e960bc699318d58966922a69
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Thu, 9 Dec 2010 14:15:34 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Thu, 16 Dec 2010 11:17:46 +0100

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>
---
kernel/sched.c | 83 +++++++++++++++++++++++++++++++++----------------------
1 files changed, 50 insertions(+), 33 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index da14302..79b557c 100644
--- 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"

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