Re: [PATCH v6 6/6] sched/topology: Restore SD_PREFER_SIBLING in domains with asymmetric capacity

On Tue, 21 Jul 2026 at 04:33, Ricardo Neri
<[email protected]> wrote:
>
> Commit 9c63e84db29b ("sched/core: Disable SD_PREFER_SIBLING on asymmetric
> CPU capacity domains") removed the SD_PREFER_SIBLING from the domains with
> asymmetric capacity. This was done to avoid spreading tasks to sibling
> scheduling groups with less capacity, but this does not happen: checks for
> capacity in update_sd_pick_busiest(), sched_balance_find_src_group(), and
> sched_balance_find_src_rq() prevent migrations from high- to low-capacity
> CPUs if the busiest group is not overloaded.
>
> The cluster topology is a notable example: some systems have scheduling
> domains spanning CPUs of asymmetric capacity, grouped into two or more
> equal-capacity clusters sharing an L2 cache. When CONFIG_SCHED_CLUSTER is
> enabled, SD_PREFER_SIBLING is needed in the domain to spread load across
> these clusters.
>
> CPUs with spare capacity, big or small, have always helped overloaded
> groups. Once the overloading condition disappears, misfit load will still
> be used to move high-utilization tasks to bigger CPUs if they have spare
> capacity.
>
> Adding the SD_PREFER_SIBLING flag shifts load balancing in shared-LLC
> domains from equalizing the number of idle CPUs to equalizing the number
> of running tasks. This enables migrations among clusters from newly-idle
> load balance, where the outgoing task is already dequeued but the CPU
> has not yet transitioned to idle.
>
> Tested-by: Christian Loehle <[email protected]>
> Tested-by: Andrea Righi <[email protected]>
> Signed-off-by: Ricardo Neri <[email protected]>

Reviewed-by: Vincent Guittot <[email protected]>


> ---
> Changes in v6:
>  * Extended the patch to keep SD_PREFER_SIBLING in all asymmetric
>    topologies. (Vincent)
>  * I removed the Reviewed-by tag from Tim, since the updated patch is
>    significantly different to what he reviewed. I am happy to re-apply
>    the tag on an updated review.
>  * Added Tested-by tag from Andrea. Thanks!
>
> Changes in v5:
>  * Improved inline comments for accuracy.
>  * Added Tested-by tag from Christian. Thanks!
>
> Changes in v4:
>  * Added Reviewed-by tag from Tim. Thanks!
>
> Changes in v3:
>  * Updated documentation of SD_PREFER_SIBLING.
>  * Expanded the patch description to explain the behavior when overloaded
>    groups are involved.
>
> Changes in v2:
>  * Reworded the patch description for clarity.
>  * Kept parentheses around bitwise operators for clarity.
> ---
>  include/linux/sched/sd_flags.h | 3 +--
>  kernel/sched/topology.c        | 4 ----
>  2 files changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> index 42839cfa2778..dc3ec2452ee1 100644
> --- a/include/linux/sched/sd_flags.h
> +++ b/include/linux/sched/sd_flags.h
> @@ -146,8 +146,7 @@ SD_FLAG(SD_ASYM_PACKING, SDF_NEEDS_GROUPS)
>  /*
>   * Prefer to place tasks in a sibling domain
>   *
> - * Set up until domains start spanning NUMA nodes. Close to being a SHARED_CHILD
> - * flag, but cleared below domains with SD_ASYM_CPUCAPACITY.
> + * Set up until domains start spanning NUMA nodes.
>   *
>   * NEEDS_GROUPS: Load balancing flag.
>   */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 622e2e01974c..21e816ad23ee 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1995,10 +1995,6 @@ sd_init(struct sched_domain_topology_level *tl,
>         /*
>          * Convert topological properties into behaviour.
>          */
> -       /* Don't attempt to spread across CPUs of different capacities. */
> -       if ((sd->flags & SD_ASYM_CPUCAPACITY) && sd->child)
> -               sd->child->flags &= ~SD_PREFER_SIBLING;
> -
>         if (sd->flags & SD_SHARE_CPUCAPACITY) {
>                 sd->imbalance_pct = 110;
>
>
> --
> 2.43.0
>