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    SubjectRe: [PATCH 5/6] cpuidle: menu: Avoid computations for very close timers
    On Thu, Oct 4, 2018 at 5:50 PM Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
    >
    > On Tue, Oct 02, 2018 at 11:46:28PM +0200, Rafael J. Wysocki wrote:
    > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    > >
    > > If the next timer event (with the tick excluded) is closer than the
    > > target residency of the second state or the PM QoS latency constraint
    > > is below its exit latency, state[0] will be used regardless of any
    > > other factors, so skip the computations in menu_select() then and
    > > return 0 straight away from it.
    > >
    > > Still, do that after the bucket has been determined to avoid
    > > disturbing the wakeup statistics in general.
    > >
    > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    > > ---
    > > drivers/cpuidle/governors/menu.c | 12 ++++++++++++
    > > 1 file changed, 12 insertions(+)
    > >
    > > Index: linux-pm/drivers/cpuidle/governors/menu.c
    > > ===================================================================
    > > --- linux-pm.orig/drivers/cpuidle/governors/menu.c
    > > +++ linux-pm/drivers/cpuidle/governors/menu.c
    > > @@ -309,6 +309,18 @@ static int menu_select(struct cpuidle_dr
    > > get_iowait_load(&nr_iowaiters, &cpu_load);
    > > data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
    > >
    > > + if (unlikely(drv->state_count <= 1) ||
    >
    > I'm not sure this test is necessary.

    Yes, it is IMO.

    Strictly speaking it prevents state[1] from being accessed if the
    count is not 2 at least.

    > If state_count == 0, we don't have to select anything as the cpuidle can't register because of:
    >
    > static int __cpuidle_register_driver(struct cpuidle_driver *drv)
    > {
    > int ret;
    >
    > if (!drv || !drv->state_count)
    > return -EINVAL;
    >
    > [ ... ]
    > }
    >
    > If state_count == 1, then there is nothing to select, it is always the state 0.

    Which is why it is better to simply return 0 right away in this case. :-)

    I guess I could compare state_count just to 1, but <= 1 works too.

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