Messages in this thread Patch in this message | | | Date | Wed, 1 Jan 2020 22:13:29 +0800 | From | Peng Liu <> | Subject | Re: [PATCH] sched/fair: fix sgc->{min,max}_capacity miscalculate |
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On Wed, Jan 01, 2020 at 05:56:49AM +0000, Valentin Schneider wrote: > 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!
Valentin, thanks for your time!
> > > 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;
sgc->capacity is the *sum* of all CPU's capacity in that group, right? {min,max}_capacity are per CPU variables(*part* of a group). So we can't compare *part* to *sum*. Am I overlooking something? Thanks.
> + > + 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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