linux-kernel - Re: [PATCH v2] sched: rt: Make RT capacity aware

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Message-ID: <39c08971-5d07-8018-915b-9c6284f89d5d@arm.com>
Date:   Wed, 30 Oct 2019 12:57:09 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware

On 09.10.19 12:46, Qais Yousef wrote:

[...]

> Changes in v2:
> 	- Use cpupri_find() to check the fitness of the task instead of
> 	  sprinkling find_lowest_rq() with several checks of
> 	  rt_task_fits_capacity().
> 
> 	  The selected implementation opted to pass the fitness function as an
> 	  argument rather than call rt_task_fits_capacity() capacity which is
> 	  a cleaner to keep the logical separation of the 2 modules; but it
> 	  means the compiler has less room to optimize rt_task_fits_capacity()
> 	  out when it's a constant value.

I would prefer exporting rt_task_fits_capacity() sched-internally via
kernel/sched/sched.h. Less code changes and the indication whether
rt_task_fits_capacity() has to be used in cpupri_find() is already given
by lowest_mask being !NULL or NULL.

[...]

> +inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> +{
> +	unsigned int min_cap;
> +	unsigned int max_cap;
> +	unsigned int cpu_cap;

Nit picking. Since we discussed it already,

I found this "Also please try to aggregate variables of the same type
into a single line. There is no point in wasting screen space::" ;-)

https://lore.kernel.org/r/20181107171149.165693799@linutronix.de

[...]

> @@ -2223,7 +2273,10 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>  	 */
>  	if (task_on_rq_queued(p) && rq->curr != p) {
>  #ifdef CONFIG_SMP
> -		if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
> +		bool need_to_push = rq->rt.overloaded ||
> +				    !rt_task_fits_capacity(p, cpu_of(rq));
> +
> +		if (p->nr_cpus_allowed > 1 && need_to_push)
>  			rt_queue_push_tasks(rq);
>  #endif /* CONFIG_SMP */
>  		if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
What happens to a always running CFS task which switches to RT? Luca
introduced a special migrate callback (next to push and pull)
specifically to deal with this scenario. A lot of new infrastructure for
this one use case, but still, do we care for it in RT as well?

https://lore.kernel.org/r/20190506044836.2914-4-luca.abeni@santannapisa.it