sched/fair: Fix value reported by hot tasks pulled in /proc/schedstat

JIRA: https://issues.redhat.com/browse/RHEL-23495

commit a430d99
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Fri Dec 20 06:32:19 2024 +0000

    sched/fair: Fix value reported by hot tasks pulled in /proc/schedstat

    In /proc/schedstat, lb_hot_gained reports the number hot tasks pulled
    during load balance. This value is incremented in can_migrate_task()
    if the task is migratable and hot. After incrementing the value,
    load balancer can still decide not to migrate this task leading to wrong
    accounting. Fix this by incrementing stats when hot tasks are detached.
    This issue only exists in detach_tasks() where we can decide to not
    migrate hot task even if it is migratable. However, in detach_one_task(),
    we migrate it unconditionally.

    [Swapnil: Handled the case where nr_failed_migrations_hot was not accounted properly and wrote commit log]

    Fixes: d319808 ("sched: Move up affinity check to mitigate useless redoing overhead")
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reported-by: "Gautham R. Shenoy" <gautham.shenoy@amd.com>
    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/20241220063224.17767-2-swapnil.sapkal@amd.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

include/linux/sched.h

@@ -911,6 +911,7 @@ struct task_struct {
 	unsigned			sched_reset_on_fork:1;
 	unsigned			sched_contributes_to_load:1;
 	unsigned			sched_migrated:1;
+	unsigned			sched_task_hot:1;
 
 	/* Force alignment to the next boundary: */
 	unsigned			:0;

kernel/sched/fair.c

@@ -8904,6 +8904,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	int tsk_cache_hot;
 
 	lockdep_assert_rq_held(env->src_rq);
+	if (p->sched_task_hot)
+		p->sched_task_hot = 0;
 
 	/*
 	 * We do not migrate tasks that are:
@@ -8976,10 +8978,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 
 	if (tsk_cache_hot <= 0 ||
 	    env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
-		if (tsk_cache_hot == 1) {
-			schedstat_inc(env->sd->lb_hot_gained[env->idle]);
-			schedstat_inc(p->stats.nr_forced_migrations);
-		}
+		if (tsk_cache_hot == 1)
+			p->sched_task_hot = 1;
 		return 1;
 	}
 
@@ -8994,6 +8994,12 @@ static void detach_task(struct task_struct *p, struct lb_env *env)
 {
 	lockdep_assert_rq_held(env->src_rq);
 
+	if (p->sched_task_hot) {
+		p->sched_task_hot = 0;
+		schedstat_inc(env->sd->lb_hot_gained[env->idle]);
+		schedstat_inc(p->stats.nr_forced_migrations);
+	}
+
 	deactivate_task(env->src_rq, p, DEQUEUE_NOCLOCK);
 	set_task_cpu(p, env->dst_cpu);
 }
@@ -9154,6 +9160,9 @@ static int detach_tasks(struct lb_env *env)
 
 		continue;
 next:
+		if (p->sched_task_hot)
+			schedstat_inc(p->stats.nr_failed_migrations_hot);
+
 		list_move(&p->se.group_node, tasks);
 	}