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sched_ext: Reject oversized automatic CID shards

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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.

tj: The bug is only reproducible with artificial VM topologies. It can't
    happen on real hardware.

Fixes: e2c841b91b ("sched_ext: Add CID sharding")
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Hui Su 2026-10-21 18:12:14 +09:00 committed by Tejun Heo
parent 3b1c7c74a6
commit 651694bc47
1 changed files with 6 additions and 0 deletions
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6
kernel/sched/ext/cid.c
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@ -309,6 +309,12 @@ 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 ||