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SubjectRe: [PATCH -v2 15/18] sched/fair: Align PELT windows between cfs_rq and its se
On Wed, Oct 04, 2017 at 08:27:01PM +0100, Dietmar Eggemann wrote:
> On 01/09/17 14:21, Peter Zijlstra wrote:
> > The PELT _sum values are a saw-tooth function, dropping on the decay
> > edge and then growing back up again during the window.
> >
> > When these window-edges are not aligned between cfs_rq and se, we can
> > have the situation where, for example, on dequeue, the se decays
> > first.
> >
> > Its _sum values will be small(er), while the cfs_rq _sum values will
> > still be on their way up. Because of this, the subtraction:
> > cfs_rq->avg._sum -= se->avg._sum will result in a positive value. This
> > will then, once the cfs_rq reaches an edge, translate into its _avg
> > value jumping up.
> >
> > This is especially visible with the runnable_load bits, since they get
> > added/subtracted a lot.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++--------------
> > 1 file changed, 31 insertions(+), 14 deletions(-)
>
> [...]
>
> > @@ -3644,7 +3634,34 @@ update_cfs_rq_load_avg(u64 now, struct c
> > */
> > static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > {
> > + u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
> > +
> > + /*
> > + * When we attach the @se to the @cfs_rq, we must align the decay
> > + * window because without that, really weird and wonderful things can
> > + * happen.
> > + *
> > + * XXX illustrate
> > + */
> > se->avg.last_update_time = cfs_rq->avg.last_update_time;
> > + se->avg.period_contrib = cfs_rq->avg.period_contrib;
> > +
> > + /*
> > + * Hell(o) Nasty stuff.. we need to recompute _sum based on the new
> > + * period_contrib. This isn't strictly correct, but since we're
> > + * entirely outside of the PELT hierarchy, nobody cares if we truncate
> > + * _sum a little.
> > + */
> > + se->avg.util_sum = se->avg.util_avg * divider;
> > +
> > + se->avg.load_sum = divider;
> > + if (se_weight(se)) {
> > + se->avg.load_sum =
> > + div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
> > + }
>
> Can scale_load_down(se->load.weight) ever become 0 here?

Yeah, don't see why not.

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