Re: [PATCH 2/3] sched/fair: Make cfs_rq::idle the sum of ancestor tg::idle

[Peter Zijlstra]

On Fri, Nov 06, 2026 at 09:31:06AM +0100, Peter Zijlstra wrote:
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -15594,8 +15594,10 @@ static void set_next_task_fair(struct rq
> > >  		hrtick_start_fair(rq, p);
> > >  }
> > >  
> > > -void init_cfs_rq(struct cfs_rq *cfs_rq)
> > > +void init_cfs_rq(struct cfs_rq *cfs_rq, struct cfs_rq *parent)
> > >  {
> > > +	if (parent)
> > > +		cfs_rq->idle = parent->idle;
> > 
> > AFAICT this creates a race:
> 
> > Or am I missing something?
> 
> Nah, you're right, this needs rq->lock.

Something like so..

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8970,7 +8970,7 @@ void __init sched_init(void)
 		rq->nr_running = 0;
 		rq->calc_load_active = 0;
 		rq->calc_load_update = jiffies + LOAD_FREQ;
-		init_cfs_rq(&rq->cfs, NULL);
+		init_cfs_rq(&rq->cfs);
 		init_rt_rq(&rq->rt);
 		init_dl_rq(&rq->dl);
 #ifdef CONFIG_FAIR_GROUP_SCHED
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -15594,10 +15594,8 @@ static void set_next_task_fair(struct rq
 		hrtick_start_fair(rq, p);
 }
 
-void init_cfs_rq(struct cfs_rq *cfs_rq, struct cfs_rq *parent)
+void init_cfs_rq(struct cfs_rq *cfs_rq)
 {
-	if (parent)
-		cfs_rq->idle = parent->idle;
 	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
 	cfs_rq->zero_vruntime = (u64)(-(1LL << 20));
 	raw_spin_lock_init(&cfs_rq->removed.lock);
@@ -15648,7 +15646,7 @@ int alloc_fair_sched_group(struct task_g
 			goto err;
 
 		se = tg_se(tg, i);
-		init_cfs_rq(cfs_rq, tg_cfs_rq(parent, i));
+		init_cfs_rq(cfs_rq);
 		init_tg_cfs_entry(tg, cfs_rq, se, i, tg_se(parent, i));
 		init_entity_runnable_average(se);
 	}
@@ -15661,19 +15659,20 @@ int alloc_fair_sched_group(struct task_g
 
 void online_fair_sched_group(struct task_group *tg)
 {
-	struct sched_entity *se;
-	struct rq_flags rf;
-	struct rq *rq;
 	int i;
 
 	for_each_possible_cpu(i) {
-		rq = cpu_rq(i);
-		se = tg_se(tg, i);
-		rq_lock_irq(rq, &rf);
+		struct cfs_rq *cfs_rq = tg_cfs_rq(tg, i);
+		struct sched_entity *se = tg_se(tg, i);
+		struct rq *rq = cpu_rq(i);
+
+		guard(rq_lock_irq)(rq);
+
 		update_rq_clock(rq);
 		attach_entity_cfs_rq(se);
 		sync_throttle(tg, i);
-		rq_unlock_irq(rq, &rf);
+
+		cfs_rq->idle = tg_cfs_rq(tg->parent, i)->idle;
 	}
 }