Messages in this thread | | | Date | Wed, 03 Sep 2014 17:51:45 +0530 | From | Preeti U Murthy <> | Subject | [QUERY] Confusing usage of rq->nr_running in load balancing |
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Hi,
There are places in kernel/sched/fair.c in the load balancing part where rq->nr_running is used as against cfs_rq->nr_running. At least I could not make out why the former was used in the following scenarios. It looks to me that it can very well lead to incorrect load balancing. Also I did not pay attention to the numa balancing part of the code while skimming through this file to catch this scenario. There are a couple of places there too which need to be scrutinized.
1. load_balance(): The check (busiest->nr_running > 1) The load balancing would be futile if there are tasks of other scheduling classes, wouldn't it?
2. active_load_balance_cpu_stop(): A similar check and a similar consequence as 1 here.
3. nohz_kick_needed() : We check for more than one task on the runqueue and hence trigger load balancing even if there are rt-tasks.
4. cpu_avg_load_per_task(): This stands out among the rest as an incorrect usage of rq->nr_running in place of cfs_rq->nr_running. We divide the load associated with the cfs_rq by the number of tasks on the rq. This will make the cfs_rq load look smaller.
5. task_hot() : I am not too sure about the consequences of using rq->nr_running here.
6. update_sg_lb_stats(): sgs->sum_nr_running is the sum of rq->nr_running and propogates thus throughout the load balancing code path.
7. sg_capacity_factor(): Returns the capacity factor measured against the cpu capacity available to fair tasks. We then compare this with the rq->nr_running in update_sg_lb_stats(), update_sd_pick_busiest() and calculate_imbalance()
8. find_busiest_queue(): This anomaly shows up when we filter against rq->nr_running == 1 and imbalance cannot be taken care of by the existing task on this rq.
Did I miss something or is it true that the usage of rq->nr_running in the above places is incorrect?
Thanks
Regards Preeti U Murthy
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