linux-kernel - Re: [PATCH 3/5] sched/deadline: Use __sub_running_bw() throughout dl_change

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Message-ID: <fc48cf4a-6153-5ead-72cd-76df4b10d6a0@arm.com>
Date:   Mon, 29 Jul 2019 18:21:55 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Qais Yousef <Qais.Yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] sched/deadline: Use __sub_running_bw() throughout
 dl_change_utilization()

On 7/29/19 5:47 PM, Peter Zijlstra wrote:
> On Fri, Jul 26, 2019 at 09:27:54AM +0100, Dietmar Eggemann wrote:
>> dl_change_utilization() has a BUG_ON() to check that no schedutil
>> kthread (sugov) is entering this function. So instead of calling
>> sub_running_bw() which checks for the special entity related to a
>> sugov thread, call the underlying function __sub_running_bw().
>>
>> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
>> ---
>>  kernel/sched/deadline.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 99d4c24a8637..1fa005f79307 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -164,7 +164,7 @@ void dl_change_utilization(struct task_struct *p, u64 new_bw)
>>  
>>  	rq = task_rq(p);
>>  	if (p->dl.dl_non_contending) {
>> -		sub_running_bw(&p->dl, &rq->dl);
>> +		__sub_running_bw(p->dl.dl_bw, &rq->dl);
>>  		p->dl.dl_non_contending = 0;
>>  		/*
>>  		 * If the timer handler is currently running and the
> 
> I'm confused; the only called of dl_change_utilization() is
> sched_dl_overflow(), and that already checks FLAG_SUGOV and exits before
> calling.

That's right. There is even a BUG_ON(p->dl.flags & SCHED_FLAG_SUGOV) at
the beginning of dl_change_utilization().

> So how can this matter?

We save the 'if (!dl_entity_is_special(dl_se))' from sub_running_bw()
when we call __sub_running_bw() since we know it can't be a special task.

Later in dl_change_utilization() we already use the inner bw accounting
functions __sub_rq_bw() and __add_rq_bw().