linux-kernel - Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle

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Date:   Fri, 9 Feb 2018 13:53:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rohit Jain <rohit.k.jain@...cle.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        joelaf@...gle.com, dietmar.eggemann@....com,
        vincent.guittot@...aro.org, morten.rasmussen@....com,
        eas-dev@...ts.linaro.org
Subject: Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in
 select_idle_sibling

On Mon, Jan 29, 2018 at 03:27:09PM -0800, Rohit Jain wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 26a71eb..ce5ccf8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu)
>  	return cpu_rq(cpu)->cpu_capacity_orig;
>  }
>  
> +static inline bool full_capacity(int cpu)
> +{
> +	return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
> +}

I don't like that name; >.75 != 1.

Maybe invert things and do something like:

static inline bool reduced_capacity(int cpu)
{
	return capacity_of(cpu) < (3*capacity_orig_of(cpu))/4;
}

> @@ -6110,11 +6116,13 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>  	for_each_cpu(cpu, cpu_smt_mask(target)) {
>  		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>  			continue;
> +		if (idle_cpu(cpu) && (capacity_of(cpu) > max_cap)) {
> +			max_cap = capacity_of(cpu);
> +			rcpu = cpu;
> +		}

		if (idle_cpu(cpu)) {
			if (!reduced_capacity(cpu))
				return cpu;

			if (capacity_cpu(cpu) > max_cap) {
				max_cap = capacity_cpu(cpu);
				rcpu = cpu;
			}
		}

Would be more consistent, I think.

>  	}
>  
> -	return -1;
> +	return rcpu;
>  }



> @@ -6143,6 +6151,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	u64 time, cost;
>  	s64 delta;
>  	int cpu, nr = INT_MAX;
> +	int best_cpu = -1;
> +	unsigned int best_cap = 0;

Randomly different names for the same thing as in select_idle_smt().
Thinking up two different names for the same thing is more work; be more
lazy.

>  	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>  	if (!this_sd)
> @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  			return -1;
>  		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>  			continue;
> +		if (idle_cpu(cpu)) {
> +			if (full_capacity(cpu)) {
> +				best_cpu = cpu;
> +				break;
> +			} else if (capacity_of(cpu) > best_cap) {
> +				best_cap = capacity_of(cpu);
> +				best_cpu = cpu;
> +			}
> +		}

No need for the else. And you'll note you're once again inconsistent
with your previous self.

But here I worry about big.little a wee bit. I think we're allowed big
and little cores on the same L3 these days, and you can't directly
compare capacity between them.

Morten / Dietmar, any comments?

> @@ -6193,13 +6210,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  	struct sched_domain *sd;
>  	int i;
>  
> -	if (idle_cpu(target))
> +	if (idle_cpu(target) && full_capacity(target))
>  		return target;
>  
>  	/*
>  	 * If the previous cpu is cache affine and idle, don't be stupid.
>  	 */
> -	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
> +	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev) &&
> +	    full_capacity(prev))
>  		return prev;

split before idle_cpu() for a better balance.