[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, 17 insertions, 15 deletions

diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index 4c621fc3c3b..640d6908f8e 100644
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -197,11 +197,15 @@ void
 default_idle (void)
 {
 	local_irq_enable();
-	while (!need_resched())
-		if (can_do_pal_halt)
-			safe_halt();
-		else
+	while (!need_resched()) {
+		if (can_do_pal_halt) {
+			local_irq_disable();
+			if (!need_resched())
+				safe_halt();
+			local_irq_enable();
+		} else
 			cpu_relax();
+	}
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -263,16 +267,16 @@ void __attribute__((noreturn))
 cpu_idle (void)
 {
 	void (*mark_idle)(int) = ia64_mark_idle;
+  	int cpu = smp_processor_id();
+	set_thread_flag(TIF_POLLING_NRFLAG);
 
 	/* endless idle loop with no priority at all */
 	while (1) {
+		if (!need_resched()) {
+			void (*idle)(void);
 #ifdef CONFIG_SMP
-		if (!need_resched())
 			min_xtp();
 #endif
-		while (!need_resched()) {
-			void (*idle)(void);
-
 			if (__get_cpu_var(cpu_idle_state))
 				__get_cpu_var(cpu_idle_state) = 0;
 
@@ -284,19 +288,17 @@ cpu_idle (void)
 			if (!idle)
 				idle = default_idle;
 			(*idle)();
-		}
-
-		if (mark_idle)
-			(*mark_idle)(0);
-
+			if (mark_idle)
+				(*mark_idle)(0);
 #ifdef CONFIG_SMP
-		normal_xtp();
+			normal_xtp();
 #endif
+		}
 		preempt_enable_no_resched();
 		schedule();
 		preempt_disable();
 		check_pgt_cache();
-		if (cpu_is_offline(smp_processor_id()))
+		if (cpu_is_offline(cpu))
 			play_dead();
 	}
 }