Re: [PATCH 1/2] sched/fair: Fix value reported by hot tasks pulled in /proc/schedstat

[Peter Zijlstra]

On Wed, Jun 14, 2023 at 10:22:23AM +0000, Swapnil Sapkal wrote:
> 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 exits 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.
> 
> Fixes: d31980846f96 ("sched: Move up affinity check to mitigate useless redoing overhead")
> Reported-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
> Signed-off-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
> ---
>  kernel/sched/fair.c | 47 +++++++++++++++++++++++++++++----------------
>  1 file changed, 30 insertions(+), 17 deletions(-)

All this for just a number hardly anybody looks at :-(

Does this also work?

Please double check the order of the task_struct::sched_bitfield thing,
I've not had much wake-up juice.

---
 include/linux/sched.h |  1 +
 kernel/sched/fair.c   | 14 ++++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1292d38d66cc..eba0a78ac2a9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -887,6 +887,7 @@ 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 6189d1a45635..a88577132b20 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8569,6 +8569,8 @@ 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:
@@ -8641,10 +8643,8 @@ 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;
 	}
 
@@ -8659,6 +8659,12 @@ 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);
 }