Eric Lee / smarc-fsl-linux-kernel

09 Nov, 2005

1 commit

  • 64c7c8f88   [PATCH] sched: resched and cpu_idle rework ... Browse Code ยป

    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
    Cc: Ingo Molnar
    Cc: Con Kolivas
    Signed-off-by: Andrew Morton
    Signed-off-by: Linus Torvalds

    Nick Piggin