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

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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

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