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Subject[PATCH] [105/275] sched: Call tick_check_idle before __irq_enter
Date
2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
Commit: d267f87fb8179c6dba03d08b91952e81bc3723c7 upstream

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>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
LKML-Reference: <1286237003-12406-9-git-send-email-venki@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/sched.c | 2 +-
kernel/softirq.c | 12 +++++++++---
2 files changed, 10 insertions(+), 4 deletions(-)

Index: linux-2.6.35.y/kernel/sched.c
===================================================================
--- linux-2.6.35.y.orig/kernel/sched.c 2011-03-29 23:03:00.344291360 -0700
+++ linux-2.6.35.y/kernel/sched.c 2011-03-29 23:54:58.516505136 -0700
@@ -1884,8 +1884,8 @@

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;
/*
Index: linux-2.6.35.y/kernel/softirq.c
===================================================================
--- linux-2.6.35.y.orig/kernel/softirq.c 2011-03-29 23:03:00.249293792 -0700
+++ linux-2.6.35.y/kernel/softirq.c 2011-03-29 23:03:00.370290694 -0700
@@ -296,10 +296,16 @@

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

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