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Date: Wed, 4 May 2016 17:31:44 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Mason <clm@fb.com>, Mike Galbraith <mgalbraith@suse.de>,
        Ingo Molnar <mingo@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>, linux-kernel@vger.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads
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On Wed, May 04, 2016 at 12:37:01PM +0200, Peter Zijlstra wrote:

> +static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +	struct sched_domain *this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> +	u64 time, cost;
> +	s64 delta;
> +	int cpu, wrap;
> +
> +	if (sched_feat(AVG_CPU)) {
> +		u64 avg_idle = this_rq()->avg_idle;
> +		u64 avg_cost = this_sd->avg_scan_cost;
> +
> +		if (sched_feat(PRINT_AVG))
> +			trace_printk("idle: %Ld cost: %Ld\n", avg_idle, avg_cost);
> +
> +		if (avg_idle / 32 < avg_cost)

s/32/512/ + IDLE_SMT fixes a hackbench regression

hackbench, like tbench, doesn't like IDLE_CPU to trigger, but apparently
needs IDLE_SMT.

Bah, I could sort of explain 32 away, but 512 is firmly in the magic
value range :/

> +			return -1;
> +	}
> +
> +	time = local_clock();
> +
> +	for_each_cpu_wrap(cpu, sched_domain_span(sd), target, wrap) {
> +		if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
> +			continue;
> +		if (idle_cpu(cpu))
> +			break;
> +	}
> +
> +	time = local_clock() - time;
> +	cost = this_sd->avg_scan_cost;
> +	delta = (s64)(time - cost) / 8;
> +	/* trace_printk("time: %Ld cost: %Ld delta: %Ld\n", time, cost, delta); */
> +	this_sd->avg_scan_cost += delta;
> +
> +	return cpu;
> +}