linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Date:	Mon, 6 May 2013 01:19:30 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On Sun, May 5, 2013 at 6:45 PM, Alex Shi <alex.shi@...el.com> wrote:
> We need initialize the se.avg.{decay_count, load_avg_contrib} for a
> new forked task.
> Otherwise random values of above variables cause mess when do new task
> enqueue:
>     enqueue_task_fair
>         enqueue_entity
>             enqueue_entity_load_avg
>
> and make forking balancing imbalance since incorrect load_avg_contrib.
>
> set avg.decay_count = 0, and avg.load_avg_contrib = se->load.weight to
> resolve such issues.
>
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  kernel/sched/core.c | 6 ++++++
>  kernel/sched/fair.c | 4 ++++
>  2 files changed, 10 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c8db984..ecec7f1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1566,6 +1566,7 @@ static void __sched_fork(struct task_struct *p)
>  #ifdef CONFIG_SMP
>         p->se.avg.runnable_avg_period = 0;
>         p->se.avg.runnable_avg_sum = 0;
> +       p->se.avg.decay_count = 0;
>  #endif
>  #ifdef CONFIG_SCHEDSTATS
>         memset(&p->se.statistics, 0, sizeof(p->se.statistics));
> @@ -1653,6 +1654,11 @@ void sched_fork(struct task_struct *p)
>                 p->sched_reset_on_fork = 0;
>         }
>
> +       /* New forked task assumed with full utilization */
> +#if defined(CONFIG_SMP)
> +       p->se.avg.load_avg_contrib = p->se.load.weight;

This is missing a scale_load() right?  Further: Why not put this in
__sched_fork?

We should also charge a minimum period to make the numbers a little
more kosher, e.g.:
+       p->se.avg.runnable_avg_period = 1024;
+       p->se.avg.runnable_avg_sum = 1024;

Rather than exposing the representation of load_avg_contrib to
__sched_fork it might also be better to call:
  __update_task_entity_contrib(&p->se)
After the initialization above; this would also avoid potential bugs
like the missing scale_load() above.


> +#endif
> +
>         if (!rt_prio(p->prio))
>                 p->sched_class = &fair_sched_class;
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9c2f726..2881d42 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1508,6 +1508,10 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>          * We track migrations using entity decay_count <= 0, on a wake-up
>          * migration we use a negative decay count to track the remote decays
>          * accumulated while sleeping.
> +        *
> +        * When enqueue a new forked task, the se->avg.decay_count == 0, so
> +        * we bypass update_entity_load_avg(), use avg.load_avg_contrib initial
> +        * value: se->load.weight.
>          */
>         if (unlikely(se->avg.decay_count <= 0)) {
>                 se->avg.last_runnable_update = rq_of(cfs_rq)->clock_task;
> --
> 1.7.12
>
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