Messages in this thread Patch in this message | | | Subject | Re: [PATCH] sched/fair: fix sgc->{min,max}_capacity miscalculate | From | Valentin Schneider <> | Date | Wed, 1 Jan 2020 05:56:49 +0000 |
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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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