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Subject: Re: [PATCH 1/3] sched/rt: cpupri_find: implement fallback mechanism
 for !fit case
To:     Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Pavan Kondeti <pkondeti@codeaurora.org>
Cc:     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
References: <20200214163949.27850-1-qais.yousef@arm.com>
 <20200214163949.27850-2-qais.yousef@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <c0772fca-0a4b-c88d-fdf2-5715fcf8447b@arm.com>
Date:   Mon, 17 Feb 2020 20:09:24 +0100
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On 14/02/2020 17:39, Qais Yousef wrote:

[...]

>  /**
>   * cpupri_find - find the best (lowest-pri) CPU in the system
>   * @cp: The cpupri context
> @@ -62,80 +115,72 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
>  		struct cpumask *lowest_mask,
>  		bool (*fitness_fn)(struct task_struct *p, int cpu))
>  {
> -	int idx = 0;
>  	int task_pri = convert_prio(p->prio);
> +	int best_unfit_idx = -1;
> +	int idx = 0, cpu;
>  
>  	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
>  
>  	for (idx = 0; idx < task_pri; idx++) {
> -		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
> -		int skip = 0;
>  
> -		if (!atomic_read(&(vec)->count))
> -			skip = 1;
> -		/*
> -		 * When looking at the vector, we need to read the counter,
> -		 * do a memory barrier, then read the mask.
> -		 *
> -		 * Note: This is still all racey, but we can deal with it.
> -		 *  Ideally, we only want to look at masks that are set.
> -		 *
> -		 *  If a mask is not set, then the only thing wrong is that we
> -		 *  did a little more work than necessary.
> -		 *
> -		 *  If we read a zero count but the mask is set, because of the
> -		 *  memory barriers, that can only happen when the highest prio
> -		 *  task for a run queue has left the run queue, in which case,
> -		 *  it will be followed by a pull. If the task we are processing
> -		 *  fails to find a proper place to go, that pull request will
> -		 *  pull this task if the run queue is running at a lower
> -		 *  priority.
> -		 */
> -		smp_rmb();
> -
> -		/* Need to do the rmb for every iteration */
> -		if (skip)
> -			continue;
> -
> -		if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
> +		if (!__cpupri_find(cp, p, lowest_mask, idx))
>  			continue;
>  
> -		if (lowest_mask) {
> -			int cpu;

Shouldn't we add an extra condition here?

+               if (!static_branch_unlikely(&sched_asym_cpucapacity))
+                       return 1;
+

Otherwise non-heterogeneous systems have to got through this
for_each_cpu(cpu, lowest_mask) further below for no good reason.

> +		if (!lowest_mask || !fitness_fn)
> +			return 1;
>  
> -			cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
> +		/* Ensure the capacity of the CPUs fit the task */
> +		for_each_cpu(cpu, lowest_mask) {
> +			if (!fitness_fn(p, cpu))
> +				cpumask_clear_cpu(cpu, lowest_mask);
> +		}

[...]