sched/fair: Cleanup in migrate_degrades_locality() to improve readability

migrate_degrade_locality() would return {1, 0, -1} respectively to
indicate that migration would degrade-locality, would improve
locality, would be ambivalent to locality improvements.

This patch improves readability by changing the return value to mean:
* Any positive value degrades locality
* 0 migration doesn't affect locality
* Any negative value improves locality

[Swapnil: Fixed comments around code and wrote commit log]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Not-yet-signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241220063224.17767-3-swapnil.sapkal@amd.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8fc6648a0aa8ec26f15d8cce68ad111b2f561746..e5c0c61909b7fd9c0b73e27ebf5b7cd6d3b9dad5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9329,43 +9329,43 @@ 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 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.
  */
-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) {
@@ -9376,14 +9376,14 @@ 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;
 }
 
 #else
-static inline int migrate_degrades_locality(struct task_struct *p,
+static inline long migrate_degrades_locality(struct task_struct *p,
                                             struct lb_env *env)
 {
-       return -1;
+       return 0;
 }
 #endif
 
@@ -9393,7 +9393,7 @@ static inline int migrate_degrades_locality(struct task_struct *p,
 static
 int can_migrate_task(struct task_struct *p, struct lb_env *env)
 {
-       int tsk_cache_hot;
+       long degrades, hot;
 
        lockdep_assert_rq_held(env->src_rq);
        if (p->sched_task_hot)
@@ -9468,13 +9468,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
        if (env->flags & LBF_ACTIVE_LB)
                return 1;
 
-       tsk_cache_hot = migrate_degrades_locality(p, env);
-       if (tsk_cache_hot == -1)
-               tsk_cache_hot = task_hot(p, env);
+       degrades = migrate_degrades_locality(p, env);
+       if (!degrades)
+               hot = task_hot(p, env);
+       else
+               hot = degrades > 0;
 
-       if (tsk_cache_hot <= 0 ||
-           env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
-               if (tsk_cache_hot == 1)
+       if (!hot || env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
+               if (hot)
                        p->sched_task_hot = 1;
                return 1;
        }