[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>

arch/sparc/kernel/process.c

@@ -67,13 +67,6 @@ extern void fpsave(unsigned long *, unsigned long *, void *, unsigned long *);
 struct task_struct *last_task_used_math = NULL;
 struct thread_info *current_set[NR_CPUS];
 
-/*
- * default_idle is new in 2.5. XXX Review, currently stolen from sparc64.
- */
-void default_idle(void)
-{
-}
-
 #ifndef CONFIG_SMP
 
 #define SUN4C_FAULT_HIGH 100
@@ -92,12 +85,11 @@ void cpu_idle(void)
 			static unsigned long fps;
 			unsigned long now;
 			unsigned long faults;
-			unsigned long flags;
 
 			extern unsigned long sun4c_kernel_faults;
 			extern void sun4c_grow_kernel_ring(void);
 
-			local_irq_save(flags);
+			local_irq_disable();
 			now = jiffies;
 			count -= (now - last_jiffies);
 			last_jiffies = now;
@@ -113,13 +105,16 @@ void cpu_idle(void)
 					sun4c_grow_kernel_ring();
 				}
 			}
-			local_irq_restore(flags);
+			local_irq_enable();
 		}
 
-		while((!need_resched()) && pm_idle) {
-			(*pm_idle)();
+		if (pm_idle) {
+			while (!need_resched())
+				(*pm_idle)();
+		} else {
+			while (!need_resched())
+				cpu_relax();
 		}
-
 		preempt_enable_no_resched();
 		schedule();
 		preempt_disable();
@@ -132,15 +127,15 @@ void cpu_idle(void)
 /* This is being executed in task 0 'user space'. */
 void cpu_idle(void)
 {
+        set_thread_flag(TIF_POLLING_NRFLAG);
 	/* endless idle loop with no priority at all */
 	while(1) {
-		if(need_resched()) {
-			preempt_enable_no_resched();
-			schedule();
-			preempt_disable();
-			check_pgt_cache();
-		}
-		barrier(); /* or else gcc optimizes... */
+		while (!need_resched())
+			cpu_relax();
+		preempt_enable_no_resched();
+		schedule();
+		preempt_disable();
+		check_pgt_cache();
 	}
 }