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

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Date:	Thu, 4 Sep 2014 09:17:47 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	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 4 September 2014 01:43, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> On Wed, 2014-09-03 at 13:09 +0200, Vincent Guittot wrote:
>> 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.
>>
>
> The overload indicator is used for knowing when we can totally avoid
> load balancing to a cpu that is about to go idle.
> We can avoid load balancing when no cpu has more than 1 task.  So if you
> have say just one fair task and multiple deadline tasks on a cpu,
> and another cpu about to go idle, you should turn on normal load
> balancing in the idle path by setting overload to true.

The newly idle load balancing can only affect CFS tasks so triggering
a load_balance because a cpu is overloaded by rt tasks only, will not
change anything.

>
> So setting overload should be set based on rq->nr_running and not on
> rq->cfs.h_nr_running.

We should probably use both values like below

if ((rq->nr_running > 1) && ( rq->cfs.h_nr_running > 0))

Regards,
Vincent
>
> Thanks.
>
> Tim
>
>
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