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    • Nick Piggin's avatar
      [PATCH] sched: resched and cpu_idle rework · 64c7c8f8
      Nick Piggin authored
      
      
      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: default avatarNick Piggin <npiggin@suse.de>
      Cc: Ingo Molnar <mingo@elte.hu>
      Cc: Con Kolivas <kernel@kolivas.org>
      Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
      64c7c8f8