Messages in this thread | | | Date | Wed, 10 Sep 2014 13:51:05 +0530 | From | Preeti U Murthy <> | Subject | Re: [QUERY] Confusing usage of rq->nr_running in load balancing |
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On 09/05/2014 05:57 PM, Vincent Guittot wrote: > On 5 September 2014 14:19, Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote: >> Hi Vincent, >> >> On 09/03/2014 10:28 PM, Vincent Guittot wrote: >>> On 3 September 2014 14:21, Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote: >>>> Hi, >>> >>> Hi Preeti, >>> >>>> >>>> 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? >>> >>> agree with you >>> >>>> >>>> 2. active_load_balance_cpu_stop(): A similar check and a similar >>>> consequence as 1 here. >>> >>> agree with you >>> >>>> >>>> 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. >>> >>> I can see one potentiel reason why rq->nr_running is interesting that >>> is the group capacity might have changed because of non cfs tasks >>> since last load balance. So we need to monitor the change of the >>> groups' capacity to ensure that the average load of each group is >>> still in the same level >>> >>>> >>>> 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. >>> >>> This one is solved in the consolidation of cpu_capacity patchset >> >> Sorry, but I don't see where in your patchset you have addressed this >> issue. Can you please point out the patch? > > In [PATCH v5 03/12] sched: fix avg_load computation: > > static unsigned long cpu_avg_load_per_task(int cpu) > { > struct rq *rq = cpu_rq(cpu); > - unsigned long nr_running = ACCESS_ONCE(rq->nr_running); > + unsigned long nr_running = ACCESS_ONCE(rq->cfs.h_nr_running); > unsigned long load_avg = rq->cfs.runnable_load_avg; > > Are you referring to another problem than the one above ?
No Vincent, this is one. Thanks for pointing it out.
Regards Preeti U Murthy > > Regards > Vincent > >> >> Regards >> Preeti U Murthy >> >
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