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From: Len Brown on 28 May 2010 02:10
From: Len Brown <len.brown(a)intel.com>

commit d306ebc28649b89877a22158fe0076f06cc46f60
(ACPI: Be in TS_POLLING state during mwait based C-state entry)
fixed an important power & performance issue where ACPI c2 and c3 C-states
were clearing TS_POLLING even when using MWAIT (ACPI_STATE_FFH).
That bug had been causing us to receive redundant scheduling interrups
when we had already been woken up by MONITOR/MWAIT.

Following up on that...

In the MWAIT case, we don't have to subsequently
check need_resched(), as that c heck was there
for the TS_POLLING-clearing case.

Note that not only does the cpuidle calling function
already check need_resched() before calling us, the
low-level entry into monitor/mwait calls it twice --
guaranteeing that a write to the trigger address
can not go un-noticed.

Also, in this case, we don't have to set TS_POLLING
when we wake, because we never cleared it.

Signed-off-by: Len Brown <len.brown(a)intel.com>
Cc: Venkatesh Pallipadi <venki(a)google.com>
---
drivers/acpi/processor_idle.c | 28 ++++++++++++++++------------
1 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index 5939e7f..a4166e2 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -881,6 +881,7 @@ static int acpi_idle_enter_simple(struct cpuidle_device *dev,
return(acpi_idle_enter_c1(dev, state));

local_irq_disable();
+
if (cx->entry_method != ACPI_CSTATE_FFH) {
current_thread_info()->status &= ~TS_POLLING;
/*
@@ -888,12 +889,12 @@ static int acpi_idle_enter_simple(struct cpuidle_device *dev,
* NEED_RESCHED:
*/
smp_mb();
- }

- if (unlikely(need_resched())) {
- current_thread_info()->status |= TS_POLLING;
- local_irq_enable();
- return 0;
+ if (unlikely(need_resched())) {
+ current_thread_info()->status |= TS_POLLING;
+ local_irq_enable();
+ return 0;
+ }
}

/*
@@ -918,7 +919,8 @@ static int acpi_idle_enter_simple(struct cpuidle_device *dev,
sched_clock_idle_wakeup_event(sleep_ticks*PM_TIMER_TICK_NS);

local_irq_enable();
- current_thread_info()->status |= TS_POLLING;
+ if (cx->entry_method != ACPI_CSTATE_FFH)
+ current_thread_info()->status |= TS_POLLING;

cx->usage++;

@@ -968,6 +970,7 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
}

local_irq_disable();
+
if (cx->entry_method != ACPI_CSTATE_FFH) {
current_thread_info()->status &= ~TS_POLLING;
/*
@@ -975,12 +978,12 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
* NEED_RESCHED:
*/
smp_mb();
- }

- if (unlikely(need_resched())) {
- current_thread_info()->status |= TS_POLLING;
- local_irq_enable();
- return 0;
+ if (unlikely(need_resched())) {
+ current_thread_info()->status |= TS_POLLING;
+ local_irq_enable();
+ return 0;
+ }
}

acpi_unlazy_tlb(smp_processor_id());
@@ -1032,7 +1035,8 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
sched_clock_idle_wakeup_event(sleep_ticks*PM_TIMER_TICK_NS);

local_irq_enable();
- current_thread_info()->status |= TS_POLLING;
+ if (cx->entry_method != ACPI_CSTATE_FFH)
+ current_thread_info()->status |= TS_POLLING;

cx->usage++;

--
1.6.0.6

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Prev: [PATCH 3/8] cpuidle: make cpuidle_curr_driver static
Next: FLAT: tweak default stack alignment