On Thu, 28 Jan 2021 at 19:32, Valentin Schneider <valentin.schnei...@arm.com> wrote: > > During load-balance, groups classified as group_misfit_task are filtered > out if they do not pass > > group_smaller_max_cpu_capacity(<candidate group>, <local group>);
group_smaller_max_cpu_capacity and group_smaller_max_cpu_capacity are removed in the next patch. Merge this and the next and directly remove them > > which itself employs fits_capacity() to compare the sgc->max_capacity of > both groups. > > Due to the underlying margin, fits_capacity(X, 1024) will return false for > any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are > {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium" > CPUs, misfit migration will never intentionally upmigrate it to a CPU of > higher capacity due to the aforementioned margin. > > One may argue the 20% margin of fits_capacity() is excessive in the advent > of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here > is that fits_capacity() is meant to compare a utilization value to a > capacity value, whereas here it is being used to compare two capacity > values. As CPU capacity and task utilization have different dynamics, a > sensible approach here would be to add a new helper dedicated to comparing > CPU capacities. > > Introduce capacity_greater(), which uses a 5% margin. Use it to replace the > existing capacity checks. Note that check_cpu_capacity() uses yet another > margin (sd->imbalance_pct), and is left alone for now. > > Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com> > --- > kernel/sched/fair.c | 15 +++++++++++---- > 1 file changed, 11 insertions(+), 4 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7d01fa0bfc7e..58ce0b22fcb0 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu) > */ > #define fits_capacity(cap, max) ((cap) * 1280 < (max) * 1024) > > +/* > + * The margin used when comparing CPU capacities. > + * is 'cap' noticeably greater than 'ref' > + * > + * (default: ~5%) > + */ > +#define capacity_greater(cap, ref) ((ref) * 1078 < (cap) * 1024) > #endif > > #ifdef CONFIG_CFS_BANDWIDTH > @@ -8253,7 +8260,7 @@ check_cpu_capacity(struct rq *rq, struct sched_domain > *sd) > static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd) > { > return rq->misfit_task_load && > - (rq->cpu_capacity_orig < rq->rd->max_cpu_capacity || > + (capacity_greater(rq->rd->max_cpu_capacity, > rq->cpu_capacity_orig) || Why do you add a margin here whereas there was no margin before ? > check_cpu_capacity(rq, sd)); > } > > @@ -8352,7 +8359,7 @@ group_is_overloaded(unsigned int imbalance_pct, struct > sg_lb_stats *sgs) > static inline bool > group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group > *ref) > { > - return fits_capacity(sg->sgc->min_capacity, ref->sgc->min_capacity); > + return capacity_greater(ref->sgc->min_capacity, > sg->sgc->min_capacity); > } > > /* > @@ -8362,7 +8369,7 @@ group_smaller_min_cpu_capacity(struct sched_group *sg, > struct sched_group *ref) > static inline bool > group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group > *ref) > { > - return fits_capacity(sg->sgc->max_capacity, ref->sgc->max_capacity); > + return capacity_greater(ref->sgc->max_capacity, > sg->sgc->max_capacity); > } > > static inline enum > @@ -9421,7 +9428,7 @@ static struct rq *find_busiest_queue(struct lb_env *env, > * average load. > */ > if (sd_has_asym_cpucapacity(env->sd) && > - capacity_of(env->dst_cpu) < capacity && > + !capacity_greater(capacity_of(env->dst_cpu), capacity) && same here > nr_running == 1) > continue; > > -- > 2.27.0 >