Re: [PATCH 09/15] sched_ext: Add reenq_flags plumbing to scx_bpf_dsq_reenq()

[Emil Tsalapatis]

On Fri Mar 6, 2026 at 2:06 PM EST, Tejun Heo wrote:
> Add infrastructure to pass flags through the deferred reenqueue path.
> reenq_local() now takes a reenq_flags parameter, and scx_sched_pcpu gains a
> deferred_reenq_local_flags field to accumulate flags from multiple
> scx_bpf_dsq_reenq() calls before processing. No flags are defined yet.
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>

> ---
>  kernel/sched/ext.c          | 33 ++++++++++++++++++++++++++++-----
>  kernel/sched/ext_internal.h | 10 ++++++++++
>  2 files changed, 38 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index b02143b10f0f..c9b0e94d59bd 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -1080,7 +1080,8 @@ static void schedule_deferred_locked(struct rq *rq)
>  	schedule_deferred(rq);
>  }
>  
> -static void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq)
> +static void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
> +			       u64 reenq_flags)
>  {
>  	/*
>  	 * Allowing reenqueues doesn't make sense while bypassing. This also
> @@ -1097,6 +1098,7 @@ static void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq
>  		scoped_guard (raw_spinlock_irqsave, &rq->scx.deferred_reenq_lock) {
>  			if (list_empty(&drl->node))
>  				list_move_tail(&drl->node, &rq->scx.deferred_reenq_locals);
> +			drl->flags |= reenq_flags;
>  		}
>  
>  		schedule_deferred(rq);
> @@ -3618,7 +3620,14 @@ int scx_check_setscheduler(struct task_struct *p, int policy)
>  	return 0;
>  }
>  
> -static u32 reenq_local(struct scx_sched *sch, struct rq *rq)
> +static bool task_should_reenq(struct task_struct *p, u64 reenq_flags)
> +{
> +	if (reenq_flags & SCX_REENQ_ANY)
> +		return true;
> +	return false;

Nit: (reenq_flags & SCX_REENQ_ANY) != 0?

> +}
> +
> +static u32 reenq_local(struct scx_sched *sch, struct rq *rq, u64 reenq_flags)
>  {
>  	LIST_HEAD(tasks);
>  	u32 nr_enqueued = 0;
> @@ -3652,6 +3661,9 @@ static u32 reenq_local(struct scx_sched *sch, struct rq *rq)
>  		if (!scx_is_descendant(task_sch, sch))
>  			continue;
>  
> +		if (!task_should_reenq(p, reenq_flags))
> +			continue;
> +
>  		dispatch_dequeue(rq, p);
>  		list_add_tail(&p->scx.dsq_list.node, &tasks);
>  	}
> @@ -3671,6 +3683,7 @@ static void process_deferred_reenq_locals(struct rq *rq)
>  
>  	while (true) {
>  		struct scx_sched *sch;
> +		u64 reenq_flags = 0;
>  
>  		scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
>  			struct scx_deferred_reenq_local *drl =
> @@ -3685,10 +3698,11 @@ static void process_deferred_reenq_locals(struct rq *rq)
>  			sch_pcpu = container_of(drl, struct scx_sched_pcpu,
>  						deferred_reenq_local);
>  			sch = sch_pcpu->sch;
> +			swap(drl->flags, reenq_flags);
>  			list_del_init(&drl->node);
>  		}
>  
> -		reenq_local(sch, rq);
> +		reenq_local(sch, rq, reenq_flags);
>  	}
>  }
>  
> @@ -7817,7 +7831,7 @@ __bpf_kfunc u32 scx_bpf_reenqueue_local(const struct bpf_prog_aux *aux)
>  	rq = cpu_rq(smp_processor_id());
>  	lockdep_assert_rq_held(rq);
>  
> -	return reenq_local(sch, rq);
> +	return reenq_local(sch, rq, 0);
>  }
>  
>  __bpf_kfunc_end_defs();
> @@ -8255,8 +8269,17 @@ __bpf_kfunc void scx_bpf_dsq_reenq(u64 dsq_id, u64 reenq_flags,
>  	if (unlikely(!sch))
>  		return;
>  
> +	if (unlikely(reenq_flags & ~__SCX_REENQ_USER_MASK)) {
> +		scx_error(sch, "invalid SCX_REENQ flags 0x%llx", reenq_flags);
> +		return;
> +	}
> +
> +	/* not specifying any filter bits is the same as %SCX_REENQ_ANY */
> +	if (!(reenq_flags & __SCX_REENQ_FILTER_MASK))
> +		reenq_flags |= SCX_REENQ_ANY;
> +
>  	dsq = find_dsq_for_dispatch(sch, this_rq(), dsq_id, smp_processor_id());
> -	schedule_dsq_reenq(sch, dsq);
> +	schedule_dsq_reenq(sch, dsq, reenq_flags);
>  }
>  
>  /**
> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index 1a8d61097cab..d9eda2e8701c 100644
> --- a/kernel/sched/ext_internal.h
> +++ b/kernel/sched/ext_internal.h
> @@ -956,6 +956,7 @@ struct scx_dsp_ctx {
>  
>  struct scx_deferred_reenq_local {
>  	struct list_head	node;
> +	u64			flags;
>  };
>  
>  struct scx_sched_pcpu {
> @@ -1128,6 +1129,15 @@ enum scx_deq_flags {
>  	SCX_DEQ_SCHED_CHANGE	= 1LLU << 33,
>  };
>  
> +enum scx_reenq_flags {
> +	/* low 16bits determine which tasks should be reenqueued */
> +	SCX_REENQ_ANY		= 1LLU << 0,	/* all tasks */
> +
> +	__SCX_REENQ_FILTER_MASK	= 0xffffLLU,
> +
> +	__SCX_REENQ_USER_MASK	= SCX_REENQ_ANY,
> +};
> +
>  enum scx_pick_idle_cpu_flags {
>  	SCX_PICK_IDLE_CORE	= 1LLU << 0,	/* pick a CPU whose SMT siblings are also idle */
>  	SCX_PICK_IDLE_IN_NODE	= 1LLU << 1,	/* pick a CPU in the same target NUMA node */