sched: Call tick_check_idle before __irq_enter

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.

Fixed-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1286237003-12406-9-git-send-email-venki@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 10 insertions, 4 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index bff9ef537df0..567f5cb9808c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1974,8 +1974,8 @@ 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 267f7b763ebb..79ee8f1fc0e7 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