Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 5 Sep 2014 14:27:36 +0200 | Subject | Re: [QUERY] Confusing usage of rq->nr_running in load balancing |
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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 ?
Regards Vincent
> > Regards > Preeti U Murthy >
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