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[PATCH] sched: resched and cpu_idle rework

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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>
This commit is contained in:
Nick Piggin 2005-11-08 21:39:04 -08:00 • committed by Linus Torvalds
parent 5bfb5d690f
commit 64c7c8f885
20 changed files with 297 additions and 221 deletions
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21
kernel/sched.c
View file

@ -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;
if (!need_resched && !nrpolling && (task_cpu(p) != smp_processor_id()))
smp_send_reschedule(task_cpu(p));
set_tsk_thread_flag(p, TIF_NEED_RESCHED);
cpu = task_cpu(p);
if (cpu == smp_processor_id())
return;
/* 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