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SubjectRe: [RESEND PATCH] sched: consider missed ticks when updating global cpu load
On Sat, Sep 26, 2015 at 03:14:45PM +0200, Frederic Weisbecker wrote:

> > when the next tick occurs, update_process_times() -> scheduler_tick()
> > -> update_cpu_load_active() is performed, assuming the distance between
> > last tick and current tick is 1 tick! it's wrong in this case. thus,
> > this abnormal case should be considered in update_cpu_load_active().
> >
> > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > ---
> > kernel/sched/fair.c | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4d5f97b..829282f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4356,12 +4356,15 @@ void update_cpu_load_nohz(void)
> > */
> > void update_cpu_load_active(struct rq *this_rq)
> > {
> > + unsigned long curr_jiffies = READ_ONCE(jiffies);
> > + unsigned long pending_updates;
> > unsigned long load = weighted_cpuload(cpu_of(this_rq));
> > /*
> > * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
> > */
> > - this_rq->last_load_update_tick = jiffies;
> > - __update_cpu_load(this_rq, load, 1);
> > + pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> > + this_rq->last_load_update_tick = curr_jiffies;
> > + __update_cpu_load(this_rq, load, pending_updates);
> > }
>
> That's right but __update_cpu_load() doesn't handle correctly pending updates
> with non-zero loads. Currently, pending updates are wheeled through decay_load_missed()
> that assume it's all about idle load.
>
> But in the cases you've enumerated, as well as in the nohz full case, missed pending
> updates can be about buzy loads.
>
> I think we need to fix update_cpu_load() to handle that first, or your fix is
> going to make things worse.

Its worse than that, the whole call chain of update_process_times()
fully assumes a single tick, fixing just the one function deep down to
handle more than 1 tick is ass backwards.


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