[PATCH v2 1/6] sched/fair: Fix value reported by hot tasks pulled in /proc/schedstat

[Swapnil Sapkal <swapnil.sapkal@amd.com>]

From: Peter Zijlstra <peterz@infradead.org>

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.

Link: https://lore.kernel.org/all/20230619092228.GK4253@hirez.programming.kicks-ass.net/

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

Fixes: d31980846f96 ("sched: Move up affinity check to mitigate useless redoing overhead")
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>
---
 include/linux/sched.h |  1 +
 kernel/sched/fair.c   | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b5916be49f62..8c6a2ed9f80e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -937,6 +937,7 @@ 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;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2c4ebfc82917..e66b10da7ac5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9390,6 +9390,8 @@ 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:
@@ -9466,10 +9468,8 @@ 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;
 	}
 
@@ -9484,6 +9484,12 @@ 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);
 }
@@ -9644,6 +9650,9 @@ 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);
 	}
 
-- 
2.43.0