linux-kernel - Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in select_idle

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Date:   Tue, 10 Oct 2017 10:54:22 -0500
From:   Atish Patra <atish.patra@...cle.com>
To:     Rohit Jain <rohit.k.jain@...cle.com>, linux-kernel@...r.kernel.org,
        eas-dev@...ts.linaro.org
Cc:     peterz@...radead.org, mingo@...hat.com, joelaf@...gle.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        morten.rasmussen@....com
Subject: Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in
 select_idle_sibling code path


Minor nit: version number missing

On 10/07/2017 06:48 PM, Rohit Jain wrote:
> While looking for CPUs to place running tasks on, the scheduler
> completely ignores the capacity stolen away by RT/IRQ tasks. This patch
> changes that behavior to also take the scaled capacity into account.
>
> Signed-off-by: Rohit Jain <rohit.k.jain@...cle.com>
> ---
>   kernel/sched/fair.c | 37 ++++++++++++++++++++++++++++---------
>   1 file changed, 28 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index eaede50..5b1f7b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6004,7 +6004,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>   
>   		for_each_cpu(cpu, cpu_smt_mask(core)) {
>   			cpumask_clear_cpu(cpu, cpus);
> -			if (!idle_cpu(cpu))
> +			if (!idle_cpu(cpu) || !full_capacity(cpu))
Do we need to skip the entire core just because 1st cpu in the core 
doesn't have full capacity ?
Let's say that is the only idle core available. It will go and try to 
select_idle_cpu() to find the idlest cpu.
Is it worth spending extra time to search an idle cpu with full capacity 
when there are idle cores available ?

Regards,
Atish
>   				idle = false;
>   		}
>   
> @@ -6025,7 +6025,8 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>    */
>   static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
>   {
> -	int cpu;
> +	int cpu, backup_cpu = -1;
> +	unsigned int backup_cap = 0;
>   
>   	if (!static_branch_likely(&sched_smt_present))
>   		return -1;
> @@ -6033,11 +6034,17 @@ 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))
> -			return cpu;
> +		if (idle_cpu(cpu)) {
> +			if (full_capacity(cpu))
> +				return cpu;
> +			if (capacity_of(cpu) > backup_cap) {
> +				backup_cap = capacity_of(cpu);
> +				backup_cpu = cpu;
> +			}
> +		}
>   	}
>   
> -	return -1;
> +	return backup_cpu;
>   }
>   
>   #else /* CONFIG_SCHED_SMT */
> @@ -6066,6 +6073,8 @@ 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 backup_cpu = -1;
> +	unsigned int backup_cap = 0;
>   
>   	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>   	if (!this_sd)
> @@ -6096,10 +6105,19 @@ 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))
> -			break;
> +		if (idle_cpu(cpu)) {
> +			if (full_capacity(cpu)) {
> +				backup_cpu = -1;
> +				break;
> +			} else if (capacity_of(cpu) > backup_cap) {
> +				backup_cap = capacity_of(cpu);
> +				backup_cpu = cpu;
> +			}
> +		}
>   	}
>   
> +	if (backup_cpu >= 0)
> +		cpu = backup_cpu;
>   	time = local_clock() - time;
>   	cost = this_sd->avg_scan_cost;
>   	delta = (s64)(time - cost) / 8;
> @@ -6116,13 +6134,14 @@ 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;
>   
>   	sd = rcu_dereference(per_cpu(sd_llc, target));