linux-kernel - Re: [QUERY] Confusing usage of rq->nr

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Date:	Fri, 05 Sep 2014 17:49:48 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	"peterz@...radead.org" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	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: Re: [QUERY] Confusing usage of rq->nr_running in load balancing

Hi Vincent,

On 09/03/2014 10:28 PM, Vincent Guittot wrote:
> On 3 September 2014 14:21, Preeti U Murthy <preeti@...ux.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?

Regards
Preeti U Murthy

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