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    SubjectRe: [PATCH] sched/fair: fix sgc->{min,max}_capacity miscalculate
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    Hi Peng,

    On 31/12/2019 03:51, Peng Liu wrote:
    > commit bf475ce0a3dd ("sched/fair: Add per-CPU min capacity to
    > sched_group_capacity") introduced per-cpu min_capacity.
    >
    > commit e3d6d0cb66f2 ("sched/fair: Add sched_group per-CPU max capacity")
    > introduced per-cpu max_capacity.
    >
    > sgc->capacity is the *SUM* of all CPU's capacity in the group.
    > sgc->{min,max}_capacity are the sg per-cpu variables. Compare with
    > sgc->capacity to get sgc->{min,max}_capacity makes no sense. Instead,
    > we should compare one by one in each iteration to get
    > sgc->{min,max}_capacity of the group.
    >

    Worth noting this only affects the SD_OVERLAP case, the other case is fine
    (I checked again just to be sure).

    Now, on the bright side of things I don't think this currently causes any
    harm. The {min,max}_capacity values are used in bits of code that only
    gets run on topologies with asymmetric CPU µarchs (SD_ASYM_CPUCAPACITY), and
    I know of no such system that is also NUMA, i.e. end up with SD_OVERLAP
    (here's hoping nobody gets any funny idea).

    Still, nice find!

    > Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
    > ---
    > kernel/sched/fair.c | 11 +++++++----
    > 1 file changed, 7 insertions(+), 4 deletions(-)
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 2d170b5da0e3..97b164fcda93 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -7795,6 +7795,7 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
    > for_each_cpu(cpu, sched_group_span(sdg)) {
    > struct sched_group_capacity *sgc;
    > struct rq *rq = cpu_rq(cpu);
    > + unsigned long cap;
    >
    > /*
    > * build_sched_domains() -> init_sched_groups_capacity()
    > @@ -7808,14 +7809,16 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
    > * causing divide-by-zero issues on boot.
    > */
    > if (unlikely(!rq->sd)) {
    > - capacity += capacity_of(cpu);
    > + cap = capacity_of(cpu);
    > + capacity += cap;
    > + min_capacity = min(cap, min_capacity);
    > + max_capacity = max(cap, max_capacity);
    > } else {
    > sgc = rq->sd->groups->sgc;
    > capacity += sgc->capacity;
    > + min_capacity = min(sgc->min_capacity, min_capacity);
    > + max_capacity = max(sgc->max_capacity, max_capacity);
    > }
    > -
    > - min_capacity = min(capacity, min_capacity);
    > - max_capacity = max(capacity, max_capacity);
    > }
    > } else {
    > /*
    >

    All that could be shortened like the below, no?

    ---
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 08a233e97a01..9f6c015639ef 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -7773,8 +7773,8 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
    */

    for_each_cpu(cpu, sched_group_span(sdg)) {
    - struct sched_group_capacity *sgc;
    struct rq *rq = cpu_rq(cpu);
    + unsigned long cpu_cap;

    /*
    * build_sched_domains() -> init_sched_groups_capacity()
    @@ -7787,15 +7787,15 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
    * This avoids capacity from being 0 and
    * causing divide-by-zero issues on boot.
    */
    - if (unlikely(!rq->sd)) {
    - capacity += capacity_of(cpu);
    - } else {
    - sgc = rq->sd->groups->sgc;
    - capacity += sgc->capacity;
    - }
    + if (unlikely(!rq->sd))
    + cpu_cap = capacity_of(cpu);
    + else
    + cpu_cap = rq->sd->groups->sgc->capacity;
    +
    + min_capacity = min(cpu_cap, min_capacity);
    + max_capacity = max(cpu_cap, max_capacity);

    - min_capacity = min(capacity, min_capacity);
    - max_capacity = max(capacity, max_capacity);
    + capacity += cpu_cap;
    }
    } else {
    /*
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