[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, 23 insertions, 14 deletions

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index 161db4acfb91..573b6a97bb1f 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -167,6 +167,19 @@ acpi_processor_power_activate(struct acpi_processor *pr,
         return;
 }
 
+static void acpi_safe_halt(void)
+{
+        int polling = test_thread_flag(TIF_POLLING_NRFLAG);
+        if (polling) {
+                clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
+        }
+        if (!need_resched())
+                safe_halt();
+        if (polling)
+                set_thread_flag(TIF_POLLING_NRFLAG);
+}
+
 static atomic_t c3_cpu_count;
 
 static void acpi_processor_idle(void)
@@ -177,7 +190,7 @@ static void acpi_processor_idle(void)
         int sleep_ticks = 0;
         u32 t1, t2 = 0;
 
-        pr = processors[raw_smp_processor_id()];
+        pr = processors[smp_processor_id()];
         if (!pr)
                 return;
 
@@ -197,8 +210,13 @@ static void acpi_processor_idle(void)
         }
 
         cx = pr->power.state;
-        if (!cx)
-                goto easy_out;
+        if (!cx) {
+                if (pm_idle_save)
+                        pm_idle_save();
+                else
+                        acpi_safe_halt();
+                return;
+        }
 
         /*
          * Check BM Activity
@@ -278,7 +296,8 @@ static void acpi_processor_idle(void)
                 if (pm_idle_save)
                         pm_idle_save();
                 else
-                        safe_halt();
+                        acpi_safe_halt();
+
                 /*
                  * TBD: Can't get time duration while in C1, as resumes
                  *      go to an ISR rather than here.  Need to instrument
@@ -414,16 +433,6 @@ static void acpi_processor_idle(void)
          */
         if (next_state != pr->power.state)
                 acpi_processor_power_activate(pr, next_state);
-
-        return;
-
-      easy_out:
-        /* do C1 instead of busy loop */
-        if (pm_idle_save)
-                pm_idle_save();
-        else
-                safe_halt();
-        return;
 }
 
 static int acpi_processor_set_power_policy(struct acpi_processor *pr)