[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, 15 insertions, 18 deletions

diff --git a/arch/sparc64/kernel/process.c b/arch/sparc64/kernel/process.c
index 2f89206e008f..02f9dec1d459 100644
--- a/arch/sparc64/kernel/process.c
+++ b/arch/sparc64/kernel/process.c
@@ -85,23 +85,31 @@ void cpu_idle(void)
 
 /*
  * the idle loop on a UltraMultiPenguin...
+ *
+ * TIF_POLLING_NRFLAG is set because we do not sleep the cpu
+ * inside of the idler task, so an interrupt is not needed
+ * to get a clean fast response.
+ *
+ * XXX Reverify this assumption... -DaveM
+ *
+ * Addendum: We do want it to do something for the signal
+ *           delivery case, we detect that by just seeing
+ *           if we are trying to send this to an idler or not.
  */
-#define idle_me_harder()        (cpu_data(smp_processor_id()).idle_volume += 1)
-#define unidle_me()             (cpu_data(smp_processor_id()).idle_volume = 0)
 void cpu_idle(void)
 {
+        cpuinfo_sparc *cpuinfo = &local_cpu_data();
         set_thread_flag(TIF_POLLING_NRFLAG);
+
         while(1) {
                 if (need_resched()) {
-                        unidle_me();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
+                        cpuinfo->idle_volume = 0;
                         preempt_enable_no_resched();
                         schedule();
                         preempt_disable();
-                        set_thread_flag(TIF_POLLING_NRFLAG);
                         check_pgt_cache();
                 }
-                idle_me_harder();
+                cpuinfo->idle_volume++;
 
                 /* The store ordering is so that IRQ handlers on
                  * other cpus see our increasing idleness for the buddy
diff --git a/arch/sparc64/kernel/smp.c b/arch/sparc64/kernel/smp.c
index 8aca4b1dc04e..797a65493fb8 100644
--- a/arch/sparc64/kernel/smp.c
+++ b/arch/sparc64/kernel/smp.c
@@ -1152,20 +1152,9 @@ void __init smp_cpus_done(unsigned int max_cpus)
                (bogosum/(5000/HZ))%100);
 }
 
-/* This needn't do anything as we do not sleep the cpu
- * inside of the idler task, so an interrupt is not needed
- * to get a clean fast response.
- *
- * XXX Reverify this assumption... -DaveM
- *
- * Addendum: We do want it to do something for the signal
- *           delivery case, we detect that by just seeing
- *           if we are trying to send this to an idler or not.
- */
 void smp_send_reschedule(int cpu)
 {
-        if (cpu_data(cpu).idle_volume == 0)
-                smp_receive_signal(cpu);
+        smp_receive_signal(cpu);
 }
 
 /* This is a nop because we capture all other cpus