• 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