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

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Date:	Wed, 3 Sep 2014 13:09:40 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 03/12] sched: fix avg_load computation

On 30 August 2014 14:00, Preeti U Murthy <preeti@...ux.vnet.ibm.com> wrote:
> 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.

Hi Preeti,

Yes, we should probably the test   rq->cfs.h_nr_running > 0 before
setting overload.

Sorry for this late answer, the email was lost in my messy inbox

Vincent

>
> 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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