linux-kernel - Re: [PATCH 2/2] sched/fair: Cleanup in migrate_degrades

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Date:   Wed, 21 Jun 2023 10:14:10 +0530
From:   Swapnil Sapkal <Swapnil.Sapkal@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, iamjoonsoo.kim@....com,
        linux-kernel@...r.kernel.org, gautham.shenoy@....com,
        kprateek.nayak@....com, wyes.karny@....com
Subject: Re: [PATCH 2/2] sched/fair: Cleanup in migrate_degrades_locality() to
 improve readability

Hello Peter,

On 6/19/2023 3:15 PM, Peter Zijlstra wrote:
> On Wed, Jun 14, 2023 at 10:22:24AM +0000, Swapnil Sapkal wrote:
>> The migrate_degrades_locality() returns tristate value whether
>> the migration will improve locality, degrades locality or no
>> impact. Handle this return values with enum to improve the
>> readability.
> 
> I can see how you ended up there, that tristate is weird, but perhaps
> don't make it more complicated than it should be?
> 
> ---
>   kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
>   1 file changed, 20 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 373ff5f55884..a8449f594348 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8446,42 +8446,42 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
>   #ifdef CONFIG_NUMA_BALANCING
>   /*
>    * Returns 1, if task migration degrades locality
> - * Returns 0, if task migration improves locality i.e migration preferred.
> - * Returns -1, if task migration is not affected by locality.
> + * Returns 0, if task migration is not affected by locality.
> + * Returns -1, if task migration improves locality i.e migration preferred.
>    */
Because of the following hunk:

> @@ -8492,14 +8492,14 @@ static int migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
>   		dst_weight = task_weight(p, dst_nid, dist);
>   	}
>   
> -	return dst_weight < src_weight;
> +	return src_weight - dst_weight;
>   }
>   

I suppose we should also change the comment to:
   /*
    * Returns a positive value, if task migration degrades locality
    * Returns 0, if task migration is not affected by locality.
    * Returns a negative value, if task migration improves locality i.e migration preferred.
    */

Do I need to resend v2 with your changes for this patchset?

> -static int migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
> +static long migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
>   {
>   	struct numa_group *numa_group = rcu_dereference(p->numa_group);
>   	unsigned long src_weight, dst_weight;
>   	int src_nid, dst_nid, dist;
>   
>   	if (!static_branch_likely(&sched_numa_balancing))
> -		return -1;
> +		return 0;
>   
>   	if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
> -		return -1;
> +		return 0;
>   
>   	src_nid = cpu_to_node(env->src_cpu);
>   	dst_nid = cpu_to_node(env->dst_cpu);
>   
>   	if (src_nid == dst_nid)
> -		return -1;
> +		return 0;
>   
>   	/* Migrating away from the preferred node is always bad. */
>   	if (src_nid == p->numa_preferred_nid) {
>   		if (env->src_rq->nr_running > env->src_rq->nr_preferred_running)
>   			return 1;
>   		else
> -			return -1;
> +			return 0;
>   	}
>   
>   	/* Encourage migration to the preferred node. */
>   	if (dst_nid == p->numa_preferred_nid)
> -		return 0;
> +		return -1;
>   
>   	/* Leaving a core idle is often worse than degrading locality. */
>   	if (env->idle == CPU_IDLE)
> -		return -1;
> +		return 0;
>   
>   	dist = node_distance(src_nid, dst_nid);
>   	if (numa_group) {
> @@ -8492,14 +8492,14 @@ static int migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
>   		dst_weight = task_weight(p, dst_nid, dist);
>   	}
>   
> -	return dst_weight < src_weight;
> +	return src_weight - dst_weight;
>   }
--
Thanks and regards,
Swapnil