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

From: Alex Shi <alex.shi@intel.com>
To: Paul Turner <pjt@google.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>,
	Namhyung Kim <namhyung@kernel.org>,
	Mike Galbraith <efault@gmx.de>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Preeti U Murthy <preeti@linux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	LKML <linux-kernel@vger.kernel.org>, Mel Gorman <mgorman@suse.de>,
	Rik van Riel <riel@redhat.com>,
	Michael Wang <wangyun@linux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task
Date: Mon, 06 May 2013 17:21:17 +0800	[thread overview]
Message-ID: <5187760D.8060900@intel.com> (raw)
In-Reply-To: <CAPM31RJg7t8f1H-dZwzP1c-0HHb_=8twidX74gS2NfyQhHUhCw@mail.gmail.com>

On 05/06/2013 04:19 PM, Paul Turner wrote:
> On Sun, May 5, 2013 at 6:45 PM, Alex Shi <alex.shi@intel.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@intel.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?

scale_load is not working now. Anyway I can add this.

> 
> 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.

Above simple change can not work.
We had talked this solution months ago. And get agreement on this patch.
https://lkml.org/lkml/2013/2/20/48  :)

-- 
Thanks
    Alex