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

[Tejun Heo <tj@kernel.org>]

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>
---
 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