linux-kernel - Re: [PATCH v9 4/8] sched/deadline: Fix bandwidth accounting at all levels after offline migration

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Message-ID: <0e546906-01b1-b15b-7188-9ad2c4b30dc1@arm.com>
Date:   Mon, 22 Jul 2019 16:29:53 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        tj@...nel.org, linux-kernel@...r.kernel.org,
        luca.abeni@...tannapisa.it, claudio@...dence.eu.com,
        tommaso.cucinotta@...tannapisa.it, bristot@...hat.com,
        mathieu.poirier@...aro.org, lizefan@...wei.com, longman@...hat.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v9 4/8] sched/deadline: Fix bandwidth accounting at all
 levels after offline migration

On 7/22/19 3:35 PM, Juri Lelli wrote:
> On 22/07/19 15:21, Dietmar Eggemann wrote:
>> On 7/22/19 2:28 PM, Juri Lelli wrote:
>>> On 22/07/19 13:07, Dietmar Eggemann wrote:
>>>> On 7/19/19 3:59 PM, Juri Lelli wrote:
>>>>
>>>> [...]
>>>>
>>>>> @@ -557,6 +558,38 @@ static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p
>>>>>  		double_lock_balance(rq, later_rq);
>>>>>  	}
>>>>>  
>>>>> +	if (p->dl.dl_non_contending || p->dl.dl_throttled) {
>>>>> +		/*
>>>>> +		 * Inactive timer is armed (or callback is running, but
>>>>> +		 * waiting for us to release rq locks). In any case, when it
>>>>> +		 * will file (or continue), it will see running_bw of this
>>>>
>>>> s/file/fire ?
>>>
>>> Yep.
>>>
>>>>> +		 * task migrated to later_rq (and correctly handle it).
>>>>
>>>> Is this because of dl_task_timer()->enqueue_task_dl()->task_contending()
>>>> setting dl_se->dl_non_contending = 0 ?
>>>
>>> No, this is related to inactive_task_timer() callback. Since the task is
>>> migrated (by this function calling set_task_cpu()) because a CPU hotplug
>>> operation happened, we need to reflect this w.r.t. running_bw, or
>>> inactive_task_timer() might sub from the new CPU and cause running_bw to
>>> underflow.
>>
>> I was more referring to the '... it will see running_bw of thus task
>> migrated to later_rq ...) and specifically to the HOW the timer
>> callback can detect this.
> 
> Oh, it actually doesn't "actively" detect this condition. The problem is
> that if it still sees dl_non_contending == 1, it will sub (from the
> "new" rq to which task's running_bw hasn't been added - w/o this fix)
> and cause the underflow.

I was wrong ... enqueue_task_dl() is called with ENQUEUE_REPLENISH which
doesn't call task_contending(). The comment makes sense to me now.