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Date: Wed, 03 Sep 2014 17:51:45 +0530 From: Preeti U Murthy <preeti@...ux.vnet.ibm.com> To: Vincent Guittot <vincent.guittot@...aro.org>, "peterz@...radead.org" <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>, Morten Rasmussen <Morten.Rasmussen@....com> CC: LKML <linux-kernel@...r.kernel.org>, Mike Galbraith <efault@....de>, Nicolas Pitre <nicolas.pitre@...aro.org>, "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>, Srikar Dronamraju <srikar@...ux.vnet.ibm.com> Subject: [QUERY] Confusing usage of rq->nr_running in load balancing 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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