Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks | Date | Thu, 28 Jan 2021 18:31:36 +0000 |
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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>);
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.schneider@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) || 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) && nr_running == 1) continue; -- 2.27.0
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