Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

Peter Zijlstra
2010-12-09T12:52:19Z

On Wed, 2010-12-08 at 15:31 -0800, Venkatesh Pallipadi wrote:

> [PATCH] update_rq_clock() with irq_time should handle 64 bit wraparound
>
> update_rq_clock with IRQ_TIME_ACCOUNTING assumed 64 bit sched_clock that
> practically will not wraparound. Change to code to handle wraparounds,
> still preventing rq->clock_task from going backwards.

All we should do is deal with the 64wrap, not filter backward motion, I
came up with something like the below.

I think for now the best solution to the early wrap problem for ARM is
for them to select HAVE_UNSTABLE_SCHED_CLOCK, it will mostly deal with
the short wrap by filtering out the occasional negative value.

Then later we can look at cleaning/breaking-up the kernel/sched_clock.c
code to provide smaller bits of functionality and possibly merging it
with some of the clocksource code.

---
kernel/sched.c | 54 +++++++++++++++++++++++++-----------------------------
1 files changed, 25 insertions(+), 29 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 8e885c1..0fb7de8 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -642,23 +642,19 @@ 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)
{
- 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;
+ delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
+ rq->clock += delta;
+ update_rq_clock_task(rq, delta);

- sched_irq_time_avg_update(rq, irq_time);
}

/*
@@ -1817,14 +1813,6 @@ void disable_sched_clock_irqtime(void)
sched_clock_irqtime = 0;
}

-static 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);
-}
-
void account_system_vtime(struct task_struct *curr)
{
unsigned long flags;
@@ -1855,25 +1843,33 @@ void account_system_vtime(struct task_struct *curr)
}
EXPORT_SYMBOL_GPL(account_system_vtime);

-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
+static inline u64 irq_time_cpu(int cpu)
{
- 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);
- }
+ return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
}

-#else
-
-static u64 irq_time_cpu(int cpu)
+static void update_rq_clock_task(struct rq *rq, s64 delta)
{
- return 0;
+ s64 irq_delta;
+
+ irq_delta = irq_time_cpu(cpu_of(rq)) - rq->prev_irq_time;
+ 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);
}

-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time) { }
+#else /* CONFIG_IRQ_TIME_ACCOUNTING */

-#endif
+static inline void update_rq_clock_task(struct rq *rq, s64 delta)
+{
+ rq->clock_task += delta;
+}
+
+#endif /* CONFIG_IRQ_TIME_ACCOUNTING */

#include "sched_idletask.c"
#include "sched_fair.c"