Re: [PATCH 2/2] sched/fair: Fix use of NULL with find_idlest_group

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 21, 2017 at 11:14:00PM +0200, Peter Zijlstra wrote:
> +static int
> +find_idlest_cpu(struct sched_domain *sd, struct task_struct *p, int cpu, int sd_flag)
> +{
> +	struct sched_domain *tmp;
> +	int new_cpu = cpu;
> +
> +	while (sd) {
> +		struct sched_group *group;
> +		int weight;
> +
> +		if (!(sd->flags & sd_flag)) {
> +			sd = sd->child;
> +			continue;
> +		}
> +
> +		group = find_idlest_group(sd, p, cpu, sd_flag);
> +		if (!group) {
> +			sd = sd->child;
> +			continue;
> +		}
> +
> +		new_cpu = find_idlest_group_cpu(group, p, cpu);
> +		if (new_cpu == -1 || new_cpu == cpu) {
> +			/* Now try balancing at a lower domain level of cpu */
> +			sd = sd->child;
> +			continue;
> +		}
> +
> +		/* Now try balancing at a lower domain level of new_cpu */
> +		cpu = new_cpu;
> +		weight = sd->span_weight;
> +		sd = NULL;
> +		for_each_domain(cpu, tmp) {
> +			if (weight <= tmp->span_weight)
> +				break;
> +			if (tmp->flags & sd_flag)
> +				sd = tmp;
> +		}

This find-the-sd-for-another-cpu thing is horrific. And it has always
bugged me that the whole thing is O(n^2) to find a CPU.

I understand why it has this form, but scanning each CPU more than once
is just offensive.

> +		/* while loop will break here if sd == NULL */
> +	}
> +
> +	return new_cpu;
> +}