Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU

From: Pavan Kondeti <pkondeti@codeaurora.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU
Date: Mon, 17 Feb 2020 14:53:29 +0530	[thread overview]
Message-ID: <20200217092329.GC28029@codeaurora.org> (raw)
In-Reply-To: <20200214163949.27850-4-qais.yousef@arm.com>

Hi Qais,

On Fri, Feb 14, 2020 at 04:39:49PM +0000, Qais Yousef wrote:

[...]

> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 0c8bac134d3a..5ea235f2cfe8 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1430,7 +1430,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  {
>  	struct task_struct *curr;
>  	struct rq *rq;
> -	bool test;
> +	bool test, fit;
>  
>  	/* For anything but wake ups, just return the task_cpu */
>  	if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK)
> @@ -1471,16 +1471,32 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	       unlikely(rt_task(curr)) &&
>  	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
>  
> -	if (test || !rt_task_fits_capacity(p, cpu)) {
> +	fit = rt_task_fits_capacity(p, cpu);
> +
> +	if (test || !fit) {
>  		int target = find_lowest_rq(p);
>  
> -		/*
> -		 * Don't bother moving it if the destination CPU is
> -		 * not running a lower priority task.
> -		 */
> -		if (target != -1 &&
> -		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
> -			cpu = target;
> +		if (target != -1) {
> +			/*
> +			 * Don't bother moving it if the destination CPU is
> +			 * not running a lower priority task.
> +			 */
> +			if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
> +
> +				cpu = target;
> +
> +			} else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
> +
> +				/*
> +				 * If the priority is the same and the new CPU
> +				 * is a better fit, then move, otherwise don't
> +				 * bother here either.
> +				 */
> +				fit = rt_task_fits_capacity(p, target);
> +				if (fit)
> +					cpu = target;
> +			}
> +		}

I understand that we are opting for the migration when priorities are tied but
the task can fit on the new task. But there is no guarantee that this task
stay there. Because any CPU that drops RT prio can pull the task. Then why
not leave it to the balancer?

I notice a case where tasks would migrate for no reason (happens without this
patch also). Assuming BIG cores are busy with other RT tasks. Now this RT
task can go to *any* little CPU. There is no bias towards its previous CPU.
I don't know if it makes any difference but I see RT task placement is too
keen on reducing the migrations unless it is absolutely needed.

Thanks,
Pavan

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