Messages in this thread Patch in this message | | | From | Jeffrey Hugo <> | Subject | [PATCH V6 RESEND] sched/fair: Remove group imbalance from calculate_imbalance() | Date | Thu, 5 Oct 2017 15:08:15 -0600 |
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The group_imbalance path in calculate_imbalance() made sense when it was added back in 2007 with commit 908a7c1b9b80 ("sched: fix improper load balance across sched domain") because busiest->load_per_task factored into the amount of imbalance that was calculated. Beginning with commit dd5feea14a7d ("sched: Fix SCHED_MC regression caused by change in sched cpu_power"), busiest->load_per_task is not a factor in the imbalance calculation, thus the group_imbalance path no longer makes sense.
The group_imbalance path can only affect the outcome of calculate_imbalance() when the average load of the domain is less than the original busiest->load_per_task. In this case, busiest->load_per_task is overwritten with the scheduling domain load average. Thus busiest->load_per_task no longer represents actual load that can be moved.
At the final comparison between env->imbalance and busiest->load_per_task, imbalance may be larger than the new busiest->load_per_task causing the check to fail under the assumption that there is a task that could be migrated to satisfy the imbalance. However env->imbalance may still be smaller than the original busiest->load_per_task, thus it is unlikely that there is a task that can be migrated to satisfy the imbalance. Calculate_imbalance() would not choose to run fix_small_imbalance() when we expect it should. In the worst case, this can result in idle cpus.
Since the group imbalance path in calculate_imbalance() is at best a NOP but otherwise harmful, remove it.
Co-authored-by: Austin Christ <austinwc@codeaurora.org> Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org> Tested-by: Tyler Baicar <tbaicar@codeaurora.org> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> ---
Peter, we were hoping you'd take this fix. The discussion last time around didn't seem ot have a specific conclusion. Please lay out how we can move forward on this. Thanks.
[v6] -Added additional history clarification to commit text
kernel/sched/fair.c | 9 --------- 1 file changed, 9 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0107280..e92a0bf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8067,15 +8067,6 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s local = &sds->local_stat; busiest = &sds->busiest_stat; - if (busiest->group_type == group_imbalanced) { - /* - * In the group_imb case we cannot rely on group-wide averages - * to ensure cpu-load equilibrium, look at wider averages. XXX - */ - busiest->load_per_task = - min(busiest->load_per_task, sds->avg_load); - } - /* * Avg load of busiest sg can be less and avg load of local sg can * be greater than avg load across all sgs of sd because avg load -- Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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