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    SubjectRe: [PATCH] sched/cfs: change initial value of runnable_avg
    On Thu, 25 Jun 2020 at 11:24, Holger Hoffstätte
    <holger@applied-asynchrony.com> wrote:
    >
    > On 2020-06-24 17:44, Vincent Guittot wrote:
    > > Some performance regression on reaim benchmark have been raised with
    > > commit 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
    > >
    > > The problem comes from the init value of runnable_avg which is initialized
    > > with max value. This can be a problem if the newly forked task is finally
    > > a short task because the group of CPUs is wrongly set to overloaded and
    > > tasks are pulled less agressively.
    > >
    > > Set initial value of runnable_avg equals to util_avg to reflect that there
    > > is no waiting time so far.
    > >
    > > Fixes: 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
    > > Reported-by: kernel test robot <rong.a.chen@intel.com>
    > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    > > ---
    > > kernel/sched/fair.c | 2 +-
    > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > >
    > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > > index 0424a0af5f87..45e467bf42fc 100644
    > > --- a/kernel/sched/fair.c
    > > +++ b/kernel/sched/fair.c
    > > @@ -806,7 +806,7 @@ void post_init_entity_util_avg(struct task_struct *p)
    > > }
    > > }
    > >
    > > - sa->runnable_avg = cpu_scale;
    > > + sa->runnable_avg = sa->util_avg;
    > >
    > > if (p->sched_class != &fair_sched_class) {
    > > /*
    > >
    >
    > Something is wrong here. I woke up my machine from suspend-to-RAM this morning
    > and saw that a completely idle machine had a loadavg of ~7. According to my

    Just to make sure: Are you speaking about loadavg that is output by
    /proc/loadavg or load_avg which is the PELT load ?
    The output of /proc/loadavg hasn't any link with runnable_avg. The 1st
    one monitors nr_running at 5 sec interval whereas the other one is a
    geometrics series of the weight of runnable tasks with a half time of
    32ms

    > monitoring system this happened to be the loadavg right before I suspended.
    > I've reverted this, rebooted, created a loadavg >0, suspended and after wake up
    > loadavg again correctly ranges between 0 and whatever, as expected.

    I'm not sure to catch why ~7 is bad compared to correctly ranges
    between 0 and whatever. Isn't ~7 part of the whatever ?

    Vincent

    >
    > -h

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