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    Subject[PATCH 3/8] sched/fair: Tweak misfit-related capacity checks
    Date
    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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