Re: [PATCH sched_ext/for-7.3] sched_ext: Set up ops.sub_ecaps_updated() dispatch context on the executing CPU

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Fri, Aug 14, 2026 at 07:46:46AM -1000, Tejun Heo wrote:
> scx_process_sync_ecaps() sets up the dispatch context for
> ops.sub_ecaps_updated() in the target cpu's pcpu context recovered from the
> llist node. However, the context is per executing cpu: the dispatch kfuncs
> resolve it with this_cpu_ptr() and the dispatch buffer lives in it. What the
> dispatches target is determined by the rq recorded in the context, not by
> which cpu's context it is. Under core scheduling the pick runs balance_one()
> for sibling rqs, so a sync processed for a sibling invokes the op with the
> executing cpu's context not set up and its dispatch kfuncs misoperate on a
> NULL or stale rq.
> 
> Set up the executing cpu's dsp_ctx instead, matching scx_dispatch_sched().
> The recorded rq keeps the dispatches targeting the synced cpu.
> 
> Fixes: b81a6c018cde ("sched_ext: Add sub_ecaps_updated() effective-cap change notifier")
> Reported-by: David Carlier <devnexen@gmail.com>
> Link: https://lore.kernel.org/all/20260813045931.8691-1-devnexen@gmail.com/
> Signed-off-by: Tejun Heo <tj@kernel.org>

Everything looks correct to me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/sched/ext/sub.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/kernel/sched/ext/sub.c
> +++ b/kernel/sched/ext/sub.c
> @@ -1017,11 +1017,15 @@ void scx_process_sync_ecaps(struct rq *r
>  		 * invocation is equivalent to the dispatch path and may drop
>  		 * and re-acquire the rq lock temporarily while the rest of
>  		 * @batch is held privately, see scx_discard_ecaps_to_sync().
> +		 * The dispatch kfuncs resolve their context on the executing
> +		 * cpu, which under core scheduling can differ from @rq's cpu,
> +		 * so the context is set up there. The rq recorded in it keeps
> +		 * the dispatches targeting @rq.
>  		 */
>  		if (ecaps != pcpu->reported_ecaps &&
>  		    SCX_HAS_OP(pcpu->sch, sub_ecaps_updated) &&
>  		    !scx_bypassing(pcpu->sch, cpu)) {
> -			struct scx_dsp_ctx *dspc = &pcpu->dsp_ctx;
> +			struct scx_dsp_ctx *dspc = &this_cpu_ptr(pcpu->sch->pcpu)->dsp_ctx;
>  
>  			dspc->rq = rq;
>  			/* stash @prev so nested dispatches can access it */
> 
> -- 
> 2.55.0