[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>

2 files changed, 6 insertions, 16 deletions

diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
index 0130f2619dac..7f8f0cda6a74 100644
--- a/arch/powerpc/platforms/iseries/setup.c
+++ b/arch/powerpc/platforms/iseries/setup.c
@@ -703,13 +703,10 @@ static void iseries_shared_idle(void)
 static void iseries_dedicated_idle(void)
 {
         long oldval;
+        set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
-                oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-                if (!oldval) {
-                        set_thread_flag(TIF_POLLING_NRFLAG);
-
+                if (!need_resched()) {
                         while (!need_resched()) {
                                 ppc64_runlatch_off();
                                 HMT_low();
@@ -722,9 +719,6 @@ static void iseries_dedicated_idle(void)
                         }
 
                         HMT_medium();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
-                } else {
-                        set_need_resched();
                 }
 
                 ppc64_runlatch_on();
diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
index 4854f5eb5c3d..a093a0d4dd69 100644
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -469,6 +469,7 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
                  * more.
                  */
                 clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
 
                 /*
                  * SMT dynamic mode. Cede will result in this thread going
@@ -481,6 +482,7 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
                         cede_processor();
                 else
                         local_irq_enable();
+                set_thread_flag(TIF_POLLING_NRFLAG);
         } else {
                 /*
                  * Give the HV an opportunity at the processor, since we are
@@ -492,11 +494,11 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
 
 static void pseries_dedicated_idle(void)
 { 
-        long oldval;
         struct paca_struct *lpaca = get_paca();
         unsigned int cpu = smp_processor_id();
         unsigned long start_snooze;
         unsigned long *smt_snooze_delay = &__get_cpu_var(smt_snooze_delay);
+        set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
                 /*
@@ -505,10 +507,7 @@ static void pseries_dedicated_idle(void)
                  */
                 lpaca->lppaca.idle = 1;
 
-                oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-                if (!oldval) {
-                        set_thread_flag(TIF_POLLING_NRFLAG);
-
+                if (!need_resched()) {
                         start_snooze = __get_tb() +
                                 *smt_snooze_delay * tb_ticks_per_usec;
 
@@ -531,9 +530,6 @@ static void pseries_dedicated_idle(void)
                         }
 
                         HMT_medium();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
-                } else {
-                        set_need_resched();
                 }
 
                 lpaca->lppaca.idle = 0;