[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 4c621fc3c3b9..640d6908f8ec 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();
         }
 }