Re: [PATCH 06/15] sched_ext: Convert deferred_reenq_locals from llist to regular list

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Mar 9, 2026 at 1:12 PM EDT, Emil Tsalapatis wrote:
> On Fri Mar 6, 2026 at 2:06 PM EST, Tejun Heo wrote:
>> The deferred reenqueue local mechanism uses an llist (lockless list) for
>> collecting schedulers that need their local DSQs re-enqueued. Convert to a
>> regular list protected by a raw_spinlock.
>>
>> The llist was used for its lockless properties, but the upcoming changes to
>> support remote reenqueue require more complex list operations that are
>> difficult to implement correctly with lockless data structures. A spinlock-
>> protected regular list provides the necessary flexibility.
>>
>> Signed-off-by: Tejun Heo <tj@kernel.org>
>
>
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
>
>> ---
>>  kernel/sched/ext.c          | 57 ++++++++++++++++++++++++-------------
>>  kernel/sched/ext_internal.h |  2 +-
>>  kernel/sched/sched.h        |  3 +-
>>  3 files changed, 41 insertions(+), 21 deletions(-)
>>
>> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
>> index 1b6cd1e4f8b9..ffccaf04e34d 100644
>> --- a/kernel/sched/ext.c
>> +++ b/kernel/sched/ext.c
>> @@ -3640,23 +3640,37 @@ static u32 reenq_local(struct scx_sched *sch, struct rq *rq)
>>  	return nr_enqueued;
>>  }
>>  
>> -static void run_deferred(struct rq *rq)
>> +static void process_deferred_reenq_locals(struct rq *rq)
>>  {
>> -	process_ddsp_deferred_locals(rq);
>> -
>> -	if (!llist_empty(&rq->scx.deferred_reenq_locals)) {
>> -		struct llist_node *llist =
>> -			llist_del_all(&rq->scx.deferred_reenq_locals);
>> -		struct scx_sched_pcpu *pos, *next;
>> +	lockdep_assert_rq_held(rq);
>>  
>> -		llist_for_each_entry_safe(pos, next, llist,
>> -					  deferred_reenq_locals_node) {
>> -			init_llist_node(&pos->deferred_reenq_locals_node);
>> -			reenq_local(pos->sch, rq);
>> +	while (true) {
>> +		struct scx_sched *sch;
>> +
>> +		scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
>> +			struct scx_sched_pcpu *sch_pcpu =
>> +				list_first_entry_or_null(&rq->scx.deferred_reenq_locals,
>> +							 struct scx_sched_pcpu,
>> +							 deferred_reenq_local_node);
>> +			if (!sch_pcpu)
>> +				return;
>> +
>> +			sch = sch_pcpu->sch;
>
> While both scx and sch_pcpu aren't used in this patch, they are useful
> for subsequent patches.
>

This comment was meant for the next patch in the series, sorry about
that. The review tag still applies.

>> +			list_del_init(&sch_pcpu->deferred_reenq_local_node);
>>  		}
>> +
>> +		reenq_local(sch, rq);
>>  	}
>>  }
>>  
>> +static void run_deferred(struct rq *rq)
>> +{
>> +	process_ddsp_deferred_locals(rq);
>> +
>> +	if (!list_empty(&rq->scx.deferred_reenq_locals))
>> +		process_deferred_reenq_locals(rq);
>> +}
>> +
>>  #ifdef CONFIG_NO_HZ_FULL
>>  bool scx_can_stop_tick(struct rq *rq)
>>  {
>> @@ -4180,13 +4194,13 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
>>  
>>  	/*
>>  	 * $sch would have entered bypass mode before the RCU grace period. As
>> -	 * that blocks new deferrals, all deferred_reenq_locals_node's must be
>> +	 * that blocks new deferrals, all deferred_reenq_local_node's must be
>>  	 * off-list by now.
>>  	 */
>>  	for_each_possible_cpu(cpu) {
>>  		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);
>>  
>> -		WARN_ON_ONCE(llist_on_list(&pcpu->deferred_reenq_locals_node));
>> +		WARN_ON_ONCE(!list_empty(&pcpu->deferred_reenq_local_node));
>>  	}
>>  
>>  	free_percpu(sch->pcpu);
>> @@ -5799,7 +5813,7 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops,
>>  		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);
>>  
>>  		pcpu->sch = sch;
>> -		init_llist_node(&pcpu->deferred_reenq_locals_node);
>> +		INIT_LIST_HEAD(&pcpu->deferred_reenq_local_node);
>>  	}
>>  
>>  	sch->helper = kthread_run_worker(0, "sched_ext_helper");
>> @@ -7126,7 +7140,8 @@ void __init init_sched_ext_class(void)
>>  		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_kick_if_idle, GFP_KERNEL, n));
>>  		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_preempt, GFP_KERNEL, n));
>>  		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_wait, GFP_KERNEL, n));
>> -		init_llist_head(&rq->scx.deferred_reenq_locals);
>> +		raw_spin_lock_init(&rq->scx.deferred_reenq_lock);
>> +		INIT_LIST_HEAD(&rq->scx.deferred_reenq_locals);
>>  		rq->scx.deferred_irq_work = IRQ_WORK_INIT_HARD(deferred_irq_workfn);
>>  		rq->scx.kick_cpus_irq_work = IRQ_WORK_INIT_HARD(kick_cpus_irq_workfn);
>>  
>> @@ -8358,7 +8373,6 @@ __bpf_kfunc void scx_bpf_reenqueue_local___v2(const struct bpf_prog_aux *aux)
>>  	unsigned long flags;
>>  	struct scx_sched *sch;
>>  	struct rq *rq;
>> -	struct llist_node *lnode;
>>  
>>  	raw_local_irq_save(flags);
>>  
>> @@ -8374,9 +8388,14 @@ __bpf_kfunc void scx_bpf_reenqueue_local___v2(const struct bpf_prog_aux *aux)
>>  		goto out_irq_restore;
>>  
>>  	rq = this_rq();
>> -	lnode = &this_cpu_ptr(sch->pcpu)->deferred_reenq_locals_node;
>> -	if (!llist_on_list(lnode))
>> -		llist_add(lnode, &rq->scx.deferred_reenq_locals);
>> +	scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
>> +		struct scx_sched_pcpu *pcpu = this_cpu_ptr(sch->pcpu);
>> +
>> +		if (list_empty(&pcpu->deferred_reenq_local_node))
>> +			list_move_tail(&pcpu->deferred_reenq_local_node,
>> +				       &rq->scx.deferred_reenq_locals);
>> +	}
>> +
>>  	schedule_deferred(rq);
>>  out_irq_restore:
>>  	raw_local_irq_restore(flags);
>> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
>> index 9e5ebd00ea0c..80d40a9c5ad9 100644
>> --- a/kernel/sched/ext_internal.h
>> +++ b/kernel/sched/ext_internal.h
>> @@ -965,7 +965,7 @@ struct scx_sched_pcpu {
>>  	 */
>>  	struct scx_event_stats	event_stats;
>>  
>> -	struct llist_node	deferred_reenq_locals_node;
>> +	struct list_head	deferred_reenq_local_node;
>>  	struct scx_dispatch_q	bypass_dsq;
>>  #ifdef CONFIG_EXT_SUB_SCHED
>>  	u32			bypass_host_seq;
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index ebe971d12cb8..0794852524e7 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -808,7 +808,8 @@ struct scx_rq {
>>  
>>  	struct task_struct	*sub_dispatch_prev;
>>  
>> -	struct llist_head	deferred_reenq_locals;
>> +	raw_spinlock_t		deferred_reenq_lock;
>> +	struct list_head	deferred_reenq_locals;	/* scheds requesting reenq of local DSQ */
>>  	struct balance_callback	deferred_bal_cb;
>>  	struct irq_work		deferred_irq_work;
>>  	struct irq_work		kick_cpus_irq_work;