linux-kernel - Re: [PATCH v5 03/12] sched: fix avg

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Date:	Sat, 30 Aug 2014 17:30:31 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org
CC:	riel@...hat.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH v5 03/12] sched: fix avg_load computation

Hi Vincent,

On 08/26/2014 04:36 PM, Vincent Guittot wrote:
> The computation of avg_load and avg_load_per_task should only takes into
> account the number of cfs tasks. The non cfs task are already taken into
> account by decreasing the cpu's capacity and they will be tracked in the
> CPU's utilization (group_utilization) of the next patches
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 87b9dc7..b85e9f7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4092,7 +4092,7 @@ static unsigned long capacity_of(int cpu)
>  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;
> 
>  	if (nr_running)
> @@ -5985,7 +5985,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  			load = source_load(i, load_idx);
> 
>  		sgs->group_load += load;
> -		sgs->sum_nr_running += rq->nr_running;
> +		sgs->sum_nr_running += rq->cfs.h_nr_running;
> 
>  		if (rq->nr_running > 1)
>  			*overload = true;
> 

Why do we probe rq->nr_running while we do load balancing? Should not we
be probing cfs_rq->nr_running instead? We are interested after all in
load balancing fair tasks right? The reason I ask this is, I was
wondering if we need to make the above similar change in more places in
load balancing.

To cite examples: The above check says a cpu is overloaded when
rq->nr_running > 1. However if these tasks happen to be rt tasks, we
would anyway not be able to load balance. So while I was looking through
this patch, I noticed this and wanted to cross verify if we are checking
rq->nr_running on purpose in some places in load balancing; another
example being in nohz_kick_needed().


Regards
Preeti U Murthy

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