[PATCH v2] sched_ext: Reject oversized automatic CID shards

[Hui Su]

Automatic topology packing starts a new shard before adding a core when
the current shard would exceed its target size. However, a first core is
always accepted so that cores remain indivisible. An indivisible core with
more than SCX_CID_SHARD_MAX_CPUS CPUs cannot be represented without
exceeding the hard shard limit.

Per-shard state uses fixed-size inline cmask storage sized for
SCX_CID_SHARD_MAX_CPUS. Passing an oversized shard to alloc_pshard() can
therefore make cmask initialization overwrite the following fields.

Reject an indivisible core larger than SCX_CID_SHARD_MAX_CPUS before adding
it to a shard. Existing non-empty shard splitting already keeps the total
within the hard cap, while an indivisible core may still exceed
cid_shard_size when it fits within the hard limit.

Report the oversized core and the hard limit with scx_error() so that the
rejection is diagnosable.

Fixes: e2c841b91bf9 ("sched_ext: Add CID sharding")
Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
Tested-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
Changes in v2:
- Report the oversized core and the hard shard limit with scx_error(),
  as suggested by Andrea Righi.

v1: https://lore.kernel.org/lkml/20261021064946.1445728-1-sh_def@xxxxxxx/
---
 kernel/sched/ext/cid.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/ext/cid.c b/kernel/sched/ext/cid.c
index 4b08866d75f6..3d0687184b44 100644
--- a/kernel/sched/ext/cid.c
+++ b/kernel/sched/ext/cid.c
@@ -287,6 +287,12 @@ s32 scx_cid_init(struct scx_sched *sch)
 				 * empty shard.
 				 */
 				cids_in_core = cpumask_weight(core_scratch);
+				if (cids_in_core > SCX_CID_SHARD_MAX_CPUS) {
+					scx_error(sch, "core with %u cpus exceeds max shard size %d",
+						  cids_in_core, SCX_CID_SHARD_MAX_CPUS);
+					return -E2BIG;
+				}
+
 				max_cores = cores_per_shard + (shard_local < nr_large ? 1 : 0);
 				if (cores_in_shard &&
 				    (cores_in_shard >= max_cores ||

base-commit: 6c377d19d4a5116d9bec5203aa3c6c11523e7898