[PATCH] sched: resched and cpu_idle rework

Make some changes to the NEED_RESCHED and POLLING_NRFLAG to reduce
confusion, and make their semantics rigid.  Improves efficiency of
resched_task and some cpu_idle routines.

* In resched_task:
- TIF_NEED_RESCHED is only cleared with the task's runqueue lock held,
  and as we hold it during resched_task, then there is no need for an
  atomic test and set there. The only other time this should be set is
  when the task's quantum expires, in the timer interrupt - this is
  protected against because the rq lock is irq-safe.

- If TIF_NEED_RESCHED is set, then we don't need to do anything. It
  won't get unset until the task get's schedule()d off.

- If we are running on the same CPU as the task we resched, then set
  TIF_NEED_RESCHED and no further action is required.

- If we are running on another CPU, and TIF_POLLING_NRFLAG is *not* set
  after TIF_NEED_RESCHED has been set, then we need to send an IPI.

Using these rules, we are able to remove the test and set operation in
resched_task, and make clear the previously vague semantics of
POLLING_NRFLAG.

* In idle routines:
- Enter cpu_idle with preempt disabled. When the need_resched() condition
  becomes true, explicitly call schedule(). This makes things a bit clearer
  (IMO), but haven't updated all architectures yet.

- Many do a test and clear of TIF_NEED_RESCHED for some reason. According
  to the resched_task rules, this isn't needed (and actually breaks the
  assumption that TIF_NEED_RESCHED is only cleared with the runqueue lock
  held). So remove that. Generally one less locked memory op when switching
  to the idle thread.

- Many idle routines clear TIF_POLLING_NRFLAG, and only set it in the inner
  most polling idle loops. The above resched_task semantics allow it to be
  set until before the last time need_resched() is checked before going into
  a halt requiring interrupt wakeup.

  Many idle routines simply never enter such a halt, and so POLLING_NRFLAG
  can be always left set, completely eliminating resched IPIs when rescheduling
  the idle task.

  POLLING_NRFLAG width can be increased, to reduce the chance of resched IPIs.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Con Kolivas <kernel@kolivas.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 23 insertions, 14 deletions

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index 161db4acfb91..573b6a97bb1f 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -167,6 +167,19 @@ acpi_processor_power_activate(struct acpi_processor *pr,
 	return;
 }
 
+static void acpi_safe_halt(void)
+{
+	int polling = test_thread_flag(TIF_POLLING_NRFLAG);
+	if (polling) {
+		clear_thread_flag(TIF_POLLING_NRFLAG);
+		smp_mb__after_clear_bit();
+	}
+	if (!need_resched())
+		safe_halt();
+	if (polling)
+		set_thread_flag(TIF_POLLING_NRFLAG);
+}
+
 static atomic_t c3_cpu_count;
 
 static void acpi_processor_idle(void)
@@ -177,7 +190,7 @@ static void acpi_processor_idle(void)
 	int sleep_ticks = 0;
 	u32 t1, t2 = 0;
 
-	pr = processors[raw_smp_processor_id()];
+	pr = processors[smp_processor_id()];
 	if (!pr)
 		return;
 
@@ -197,8 +210,13 @@ static void acpi_processor_idle(void)
 	}
 
 	cx = pr->power.state;
-	if (!cx)
-		goto easy_out;
+	if (!cx) {
+		if (pm_idle_save)
+			pm_idle_save();
+		else
+			acpi_safe_halt();
+		return;
+	}
 
 	/*
 	 * Check BM Activity
@@ -278,7 +296,8 @@ static void acpi_processor_idle(void)
 		if (pm_idle_save)
 			pm_idle_save();
 		else
-			safe_halt();
+			acpi_safe_halt();
+
 		/*
 		 * TBD: Can't get time duration while in C1, as resumes
 		 *      go to an ISR rather than here.  Need to instrument
@@ -414,16 +433,6 @@ static void acpi_processor_idle(void)
 	 */
 	if (next_state != pr->power.state)
 		acpi_processor_power_activate(pr, next_state);
-
-	return;
-
-      easy_out:
-	/* do C1 instead of busy loop */
-	if (pm_idle_save)
-		pm_idle_save();
-	else
-		safe_halt();
-	return;
 }
 
 static int acpi_processor_set_power_policy(struct acpi_processor *pr)