Messages in this thread Patch in this message | | | From | Morten Rasmussen <> | Subject | [PATCH 4/7] sched/fair: Avoid unnecessary balancing of asymmetric capacity groups | Date | Thu, 15 Feb 2018 16:20:51 +0000 |
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On systems with asymmetric cpu capacities, a skewed load distribution might yield better throughput than balancing load per group capacity. For example, preferring high capacity cpus for compute intensive tasks leaving low capacity cpus idle rather than balancing the number of idle cpus across different cpu types. Instead, let load-balance back off if the busiest group isn't really overloaded.
cc: Ingo Molnar <mingo@redhat.com> cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com> --- kernel/sched/fair.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0d70838ed711..e036aef3f10a 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7794,6 +7794,19 @@ group_smaller_cpu_capacity(struct sched_group *sg, struct sched_group *ref) ref->sgc->min_capacity * 1024; } +/* + * group_similar_cpu_capacity: Returns true if the minimum capacity of the + * compared groups differ by less than 12.5%. + */ +static inline bool +group_similar_cpu_capacity(struct sched_group *sg, struct sched_group *ref) +{ + long diff = sg->sgc->min_capacity - ref->sgc->min_capacity; + long max = max(sg->sgc->min_capacity, ref->sgc->min_capacity); + + return abs(diff) < max >> 3; +} + static inline enum group_type group_classify(struct sched_group *group, struct sg_lb_stats *sgs) @@ -7925,6 +7938,15 @@ static bool update_sd_pick_busiest(struct lb_env *env, group_smaller_cpu_capacity(sds->local, sg)) return false; + /* + * Candidate sg doesn't face any severe imbalance issues so + * don't disturb unless the groups are of similar capacity + * where balancing is more harmless. + */ + if (sgs->group_type == group_other && + !group_similar_cpu_capacity(sds->local, sg)) + return false; + asym_packing: /* This is the busiest node in its class. */ if (!(env->sd->flags & SD_ASYM_PACKING)) -- 2.7.4
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