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) <pet...@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?
[...]
