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SubjectRe: [PATCH 2/2] sched/fair: Cleanup in migrate_degrades_locality() to improve readability
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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

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