[PATCH 8/8] Call tick_check_idle before __irq_enter

Venkatesh Pallipadi
2010-10-05T00:04:41Z

When CPU is idle and on first interrupt, irq_enter calls tick_check_idle()
to notify interruption from idle. But, there is a problem if this call
is done after __irq_enter, as all routines in __irq_enter may find
stale time due to yet to be done tick_check_idle.

Specifically, trace calls in __irq_enter when they use global clock and also
account_system_vtime change in this patch as it wants to use sched_clock_cpu()
to do proper irq timing.

But, tick_check_idle was moved after __irq_enter intentionally to
prevent problem of unneeded ksoftirqd wakeups by the commit ee5f80a.
irq: call __irq_enter() before calling the tick_idle_check
Impact: avoid spurious ksoftirqd wakeups

Moving tick_check_idle() before __irq_enter and wrapping it with
local_bh_enable/disable would solve both the problems.

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
---
kernel/sched.c | 2 +-
kernel/softirq.c | 12 +++++++++---
2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 6487da4..442e151 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1983,8 +1983,8 @@ void account_system_vtime(struct task_struct *curr)

local_irq_save(flags);

- now = sched_clock();
cpu = smp_processor_id();
+ now = sched_clock_cpu(cpu);
delta = now - per_cpu(irq_start_time, cpu);
per_cpu(irq_start_time, cpu) = now;
/*
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 267f7b7..6bb7e19 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -296,10 +296,16 @@ void irq_enter(void)

rcu_irq_enter();
if (idle_cpu(cpu) && !in_interrupt()) {
- __irq_enter();
+ /*
+ * Prevent raise_softirq from needlessly waking up ksoftirqd
+ * here, as softirq will be serviced on return from interrupt.
+ */
+ local_bh_disable();
tick_check_idle(cpu);
- } else
- __irq_enter();
+ local_bh_enable();
+ }
+
+ __irq_enter();
}

#ifdef __ARCH_IRQ_EXIT_IRQS_DISABLED
--
1.7.1