Re: [PATCH sched_ext/for-7.1-fixes] tools/sched_ext: scx_qmap: Silence task_ctx lookup miss

[Andrea Righi]

Hi Tejun,

On Mon, Apr 20, 2026 at 08:13:09PM -1000, Tejun Heo wrote:
> scx_fork() dispatches ops.init_task to exactly one scheduler - the one
> owning the forking task's cgroup. A task forked inside a sub-scheduler's
> cgroup is init'd into the sub only; the root scheduler has no task_ctx
> entry for it. When that task later appears as @prev in the root's
> qmap_dispatch() (or flows through core-sched comparison via task_qdist),
> the bpf_task_storage_get() legitimately misses.
> 
> qmap treated those misses as fatal via scx_bpf_error("task_ctx lookup
> failed") and aborted the scheduler as soon as the first cross-sched
> task hit the root. Drop the error in the three sites where the miss is
> legitimate: lookup_task_ctx() (helper; callers already check for NULL),
> qmap_dispatch()'s @prev branch (bookkeeping-only), and task_qdist()
> (returns 0 which makes the comparison a no-op). The existing scx_error
> was a paranoid guard from the pre-sub-sched world where every task was
> owned by the one and only scheduler.
> 
> Fixes: 4f8b122848db ("sched_ext: Add basic building blocks for nested sub-scheduler dispatching")
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Should we return prev_cpu in qmap_select_cpu() instead of -ESRCH when
lookup_task_ctx() returns NULL? Otherwise the scheduler would error.

Apart than that looks good to me.

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  tools/sched_ext/scx_qmap.bpf.c | 22 +++++-----------------
>  1 file changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> index b68abb9e760b..0330ba15210d 100644
> --- a/tools/sched_ext/scx_qmap.bpf.c
> +++ b/tools/sched_ext/scx_qmap.bpf.c
> @@ -159,13 +159,7 @@ static s32 pick_direct_dispatch_cpu(struct task_struct *p, s32 prev_cpu)
> 
>  static struct task_ctx *lookup_task_ctx(struct task_struct *p)
>  {
> -	struct task_ctx *tctx;
> -
> -	if (!(tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0))) {
> -		scx_bpf_error("task_ctx lookup failed");
> -		return NULL;
> -	}
> -	return tctx;
> +	return bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
>  }
> 
>  s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
> @@ -540,13 +534,9 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
>  	 */
>  	if (prev) {
>  		tctx = bpf_task_storage_get(&task_ctx_stor, prev, 0, 0);
> -		if (!tctx) {
> -			scx_bpf_error("task_ctx lookup failed");
> -			return;
> -		}
> -
> -		tctx->core_sched_seq =
> -			core_sched_tail_seqs[weight_to_idx(prev->scx.weight)]++;
> +		if (tctx)
> +			tctx->core_sched_seq =
> +				core_sched_tail_seqs[weight_to_idx(prev->scx.weight)]++;
>  	}
>  }
> 
> @@ -584,10 +574,8 @@ static s64 task_qdist(struct task_struct *p)
>  	s64 qdist;
> 
>  	tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
> -	if (!tctx) {
> -		scx_bpf_error("task_ctx lookup failed");
> +	if (!tctx)
>  		return 0;
> -	}
> 
>  	qdist = tctx->core_sched_seq - core_sched_head_seqs[idx];
> 
> --
> 2.53.0