[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, 14 insertions, 7 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 0f2def822296..ac3f5cc3bb51 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -864,21 +864,28 @@ static void deactivate_task(struct task_struct *p, runqueue_t *rq)
 #ifdef CONFIG_SMP
 static void resched_task(task_t *p)
 {
-        int need_resched, nrpolling;
+        int cpu;
 
         assert_spin_locked(&task_rq(p)->lock);
 
-        /* minimise the chance of sending an interrupt to poll_idle() */
-        nrpolling = test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
-        need_resched = test_and_set_tsk_thread_flag(p,TIF_NEED_RESCHED);
-        nrpolling |= test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
+        if (unlikely(test_tsk_thread_flag(p, TIF_NEED_RESCHED)))
+                return;
+
+        set_tsk_thread_flag(p, TIF_NEED_RESCHED);
+
+        cpu = task_cpu(p);
+        if (cpu == smp_processor_id())
+                return;
 
-        if (!need_resched && !nrpolling && (task_cpu(p) != smp_processor_id()))
-                smp_send_reschedule(task_cpu(p));
+        /* NEED_RESCHED must be visible before we test POLLING_NRFLAG */
+        smp_mb();
+        if (!test_tsk_thread_flag(p, TIF_POLLING_NRFLAG))
+                smp_send_reschedule(cpu);
 }
 #else
 static inline void resched_task(task_t *p)
 {
+        assert_spin_locked(&task_rq(p)->lock);
         set_tsk_need_resched(p);
 }
 #endif