Re: [PATCH 04/15] sched_ext: Change find_global_dsq() to take CPU number instead of task

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

On Fri Mar 6, 2026 at 2:06 PM EST, Tejun Heo wrote:
> Change find_global_dsq() to take a CPU number directly instead of a task
> pointer. This prepares for callers where the CPU is available but the task is
> not.
>
> No functional changes.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  kernel/sched/ext.c | 32 +++++++++++++++-----------------
>  1 file changed, 15 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index c36d399bca3e..c44893878878 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -341,10 +341,9 @@ static bool scx_is_descendant(struct scx_sched *sch, struct scx_sched *ancestor)
>  	for ((pos) = scx_next_descendant_pre(NULL, (root)); (pos);		\
>  	     (pos) = scx_next_descendant_pre((pos), (root)))
>  
> -static struct scx_dispatch_q *find_global_dsq(struct scx_sched *sch,
> -					      struct task_struct *p)
> +static struct scx_dispatch_q *find_global_dsq(struct scx_sched *sch, s32 tcpu)

Nit: Maybe rename tcpu to cpu? It's only a task's CPU because we currently use it
that way, and the name is no longer obvious.

>  {
> -	return &sch->pnode[cpu_to_node(task_cpu(p))]->global_dsq;
> +	return &sch->pnode[cpu_to_node(tcpu)]->global_dsq;
>  }
>  
>  static struct scx_dispatch_q *find_user_dsq(struct scx_sched *sch, u64 dsq_id)
> @@ -1266,7 +1265,7 @@ static void dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
>  			scx_error(sch, "attempting to dispatch to a destroyed dsq");
>  			/* fall back to the global dsq */
>  			raw_spin_unlock(&dsq->lock);
> -			dsq = find_global_dsq(sch, p);
> +			dsq = find_global_dsq(sch, task_cpu(p));
>  			raw_spin_lock(&dsq->lock);
>  		}
>  	}
> @@ -1474,7 +1473,7 @@ static void dispatch_dequeue_locked(struct task_struct *p,
>  
>  static struct scx_dispatch_q *find_dsq_for_dispatch(struct scx_sched *sch,
>  						    struct rq *rq, u64 dsq_id,
> -						    struct task_struct *p)
> +						    s32 tcpu)
>  {
>  	struct scx_dispatch_q *dsq;
>  
> @@ -1485,20 +1484,19 @@ static struct scx_dispatch_q *find_dsq_for_dispatch(struct scx_sched *sch,
>  		s32 cpu = dsq_id & SCX_DSQ_LOCAL_CPU_MASK;
>  
>  		if (!ops_cpu_valid(sch, cpu, "in SCX_DSQ_LOCAL_ON dispatch verdict"))
> -			return find_global_dsq(sch, p);
> +			return find_global_dsq(sch, tcpu);
>  
>  		return &cpu_rq(cpu)->scx.local_dsq;
>  	}
>  
>  	if (dsq_id == SCX_DSQ_GLOBAL)
> -		dsq = find_global_dsq(sch, p);
> +		dsq = find_global_dsq(sch, tcpu);
>  	else
>  		dsq = find_user_dsq(sch, dsq_id);
>  
>  	if (unlikely(!dsq)) {
> -		scx_error(sch, "non-existent DSQ 0x%llx for %s[%d]",
> -			  dsq_id, p->comm, p->pid);
> -		return find_global_dsq(sch, p);
> +		scx_error(sch, "non-existent DSQ 0x%llx", dsq_id);
> +		return find_global_dsq(sch, tcpu);
>  	}
>  
>  	return dsq;
> @@ -1540,7 +1538,7 @@ static void direct_dispatch(struct scx_sched *sch, struct task_struct *p,
>  {
>  	struct rq *rq = task_rq(p);
>  	struct scx_dispatch_q *dsq =
> -		find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p);
> +		find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, task_cpu(p));
>  
>  	touch_core_sched_dispatch(rq, p);
>  
> @@ -1683,7 +1681,7 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
>  	dsq = &rq->scx.local_dsq;
>  	goto enqueue;
>  global:
> -	dsq = find_global_dsq(sch, p);
> +	dsq = find_global_dsq(sch, task_cpu(p));
>  	goto enqueue;
>  bypass:
>  	dsq = bypass_enq_target_dsq(sch, task_cpu(p));
> @@ -2140,7 +2138,7 @@ static struct rq *move_task_between_dsqs(struct scx_sched *sch,
>  		dst_rq = container_of(dst_dsq, struct rq, scx.local_dsq);
>  		if (src_rq != dst_rq &&
>  		    unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
> -			dst_dsq = find_global_dsq(sch, p);
> +			dst_dsq = find_global_dsq(sch, task_cpu(p));
>  			dst_rq = src_rq;
>  		}
>  	} else {
> @@ -2269,7 +2267,7 @@ static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
>  
>  	if (src_rq != dst_rq &&
>  	    unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
> -		dispatch_enqueue(sch, rq, find_global_dsq(sch, p), p,
> +		dispatch_enqueue(sch, rq, find_global_dsq(sch, task_cpu(p)), p,
>  				 enq_flags | SCX_ENQ_CLEAR_OPSS);
>  		return;
>  	}
> @@ -2407,7 +2405,7 @@ static void finish_dispatch(struct scx_sched *sch, struct rq *rq,
>  
>  	BUG_ON(!(p->scx.flags & SCX_TASK_QUEUED));
>  
> -	dsq = find_dsq_for_dispatch(sch, this_rq(), dsq_id, p);
> +	dsq = find_dsq_for_dispatch(sch, this_rq(), dsq_id, task_cpu(p));
>  
>  	if (dsq->id == SCX_DSQ_LOCAL)
>  		dispatch_to_local_dsq(sch, rq, dsq, p, enq_flags);
> @@ -2647,7 +2645,7 @@ static void process_ddsp_deferred_locals(struct rq *rq)
>  
>  		list_del_init(&p->scx.dsq_list.node);
>  
> -		dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p);
> +		dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, task_cpu(p));
>  		if (!WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL))
>  			dispatch_to_local_dsq(sch, rq, dsq, p,
>  					      p->scx.ddsp_enq_flags);
> @@ -7410,7 +7408,7 @@ static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit,
>  	}
>  
>  	/* @p is still on $src_dsq and stable, determine the destination */
> -	dst_dsq = find_dsq_for_dispatch(sch, this_rq, dsq_id, p);
> +	dst_dsq = find_dsq_for_dispatch(sch, this_rq, dsq_id, task_cpu(p));
>  
>  	/*
>  	 * Apply vtime and slice updates before moving so that the new time is