linux-kernel - Re: [RFC PATCH 2/9] sched: add search limit as per latency-nice

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Message-Id: <5b7d3790-2510-f8b1-6515-bb9d307bba25@linux.ibm.com>
Date:   Thu, 5 Sep 2019 11:52:06 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...hat.com, tglx@...utronix.de,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net, patrick.bellasi@....com
Subject: Re: [RFC PATCH 2/9] sched: add search limit as per latency-nice



On 8/30/19 11:19 PM, subhra mazumdar wrote:
> Put upper and lower limit on CPU search in select_idle_cpu. The lower limit
> is set to amount of CPUs in a core  while upper limit is derived from the
> latency-nice of the thread. This ensures for any architecture we will
> usually search beyond a core. Changing the latency-nice value by user will
> change the search cost making it appropriate for given workload.
> 
> Signed-off-by: subhra mazumdar <subhra.mazumdar@...cle.com>
> ---
>  kernel/sched/fair.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b08d00c..c31082d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	u64 avg_cost, avg_idle;
>  	u64 time, cost;
>  	s64 delta;
> -	int cpu, nr = INT_MAX;
> +	int cpu, floor, nr = INT_MAX;
> 
>  	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>  	if (!this_sd)
> @@ -6205,11 +6205,12 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  		return -1;
> 
>  	if (sched_feat(SIS_PROP)) {
> -		u64 span_avg = sd->span_weight * avg_idle;
> -		if (span_avg > 4*avg_cost)
> -			nr = div_u64(span_avg, avg_cost);
> -		else
> -			nr = 4;
> +		floor = cpumask_weight(topology_sibling_cpumask(target));
> +		if (floor < 2)
> +			floor = 2;
> +		nr = (p->latency_nice * sd->span_weight) / LATENCY_NICE_MAX;

I see you defined LATENCY_NICE_MAX = 100,
So is the value 100 an experimental value?
I was hoping to be something in the power of 2 resulting in just ">>>" rather than
the heavy division operation.

> +		if (nr < floor)
> +			nr = floor;
>  	}
> 
>  	time = local_clock();
>