Re: [PATCH v5 0/6] sched: Fix cluster scheduling in the presence of asymmetric capacity

[Andrea Righi]

Hi Ricardo,

On Mon, Jun 22, 2026 at 05:05:50PM -0700, Ricardo Neri wrote:
> Hi,
> 
> This is v5 of the patch series. The only changes are optimizations to check
> same-capacity cluster and SMT siblings only when needed. Please read the
> changelog for details.
> 
> Cluster scheduling aims to maximize performance by spreading load across
> clusters of CPUs that share mid-level resources [2]. It works well on
> uniform systems, but it breaks down on topologies with big and small
> cores arranged in clusters. As a result, it fails on several generations
> of Intel processors already shipped and upcoming.
> 
> Consider the topology below of big (B) cores and clusters of small (s)
> cores.
>          ------   ------
>          | B  |   | B  |   -----------------   -----------------
>          |    |   |    |   | s | s | s | s |   | s | s | s | s |
>          ------   ------   -----------------   -----------------
>          | L2 |   | L2 |   |      L2       |   |       L2      |
>          -------------------------------------------------------
>          |                          L3                         |
>          -------------------------------------------------------
> 
> On a partially busy system (one with idle CPUs; busy CPUs have one task
> each), scheduling for asymmetric capacity ensures that misfit tasks land on
> the big CPUs. The remaining tasks, misfit or not, run on the small CPUs.
> When CONFIG_SCHED_CLUSTER is enabled, these remaining tasks are supposed to
> be evenly spread among the small-CPU clusters. Today, this does not
> happen.
> 
> Several issues in the load balancer prevent a small CPU in one cluster
> from pulling tasks from another:
> 
>  a) update_sd_pick_busiest() may select a fully_busy group with higher
>     per-CPU capacity as the busiest, preventing a subsequent fully_busy
>     group of equal capacity from being correctly selected.
>  b) Misfit-load statistics are used to identify tasks that would benefit
>     from migrating to bigger CPUs. Accounting misfit load is pointless if
>     the destination CPU is equally small, and it also blocks balancing
>     between clusters.
>  c) Due to b), groups that are truly has_spare or fully_busy get
>     misclassified as misfit_task. update_sd_pick_busiest() then skips
>     them, since a small destination CPU cannot help with misfit tasks.
>  d) Once a busiest group has been identified, sched_balance_find_src_rq()
>     will refuse to migrate tasks to CPUs of equal capacity, even when
>     doing so is precisely what is required to balance small-CPU clusters.
>  e) The SD_PREFER_SIBLING flag is missing from scheduling domains with
>     asymmetric capacity, preventing the balancer from equalizing load
>     across sibling small-core clusters.
> 
> Together, these issues prevent cluster-level balancing on systems with
> asymmetric CPU capacity.
> 
> This series addresses each problem and restores the intended behavior.
> Details, rationale, and code changes are explained in each patch.
> 
> I tested these patches on Alder Lake, which has both SMT Pcores and
> clusters of Ecores. I tested with SMT both disabled and enabled. I also
> tested on Lunar Lake and Panther Lake, which have an Ecore cluster not
> connected to the L3 cache. I repeated the same experiment with
> CONFIG_SCHED_CLUSTER disabled. The load balancer behaves as expected.
> 
> Christian also tested this patchset on a synthetic arm64 qemu topology and
> the expected behavior [3].
> 
> Link: https://lore.kernel.org/all/20260509180955.1840064-1-arighi@xxxxxxxxxx/ [1]
> Link: https://lore.kernel.org/r/20210924085104.44806-1-21cnbao@xxxxxxxxx/ [2]
> Link: https://lore.kernel.org/all/e08492e0-d9f3-4574-8841-b633db008507@xxxxxxx/ [3]

I looked at this series and did some tests on Vera Rubin (arm64). The series
does not directly target this topology, because the system has no usable
SD_CLUSTER domain. Nevertheless, there are changes to the common fair
load-balancing and SD_ASYM_CPUCAPACITY paths, so I tested it just to check for
any potential regression.

I used DCPerf MediaWiki and a CPU-intensive SGEMM workload based on NVPL. The
results showed no measurable performance differences, with all observed
variations falling within normal run-to-run variability. Based on these results,
the series looks good on this platform as well.

Tested-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> 
> Changes in v5:
>  - Added Tested-by tags from Christian. Thanks!
>  - Patch 1 (pre-work): Optimized logic to identify CPUs with busy SMT
>    siblings only when needed. (Prateek, Chen Yu)
>  - Patch 5: Optimized logic to check for architectural capacity only when
>    needed.
>  - Added Reviewed-by tag from Prateek. Thanks!
>  - Link to v4: https://lore.kernel.org/r/20260608-rneri-fix-cas-clusters-v4-0-1526711c944c@xxxxxxxxxxxxxxx
> 
> Changes in v4:
>  - Patch 1 (pre-work): Fixed a bug that would block load balancing on SMT
>    cores with more than one busy sibling.
>  - Patch 2 (pre-work): Fixed a bug that would needlessly update
>    sg_overloaded.
>  - Patch 5: Reworked logic using a local variable for improved
>    readability.
>  - Added Reviewed-by tags from Chen Yu, Tim, and Vincent. Thanks!
>  - Link to v3: https://lore.kernel.org/r/20260514-rneri-fix-cas-clusters-v3-0-0037869554bd@xxxxxxxxxxxxxxx
> 
> Changes in v3:
>  - Patch 3: Reverted the inverted runtime capacity check. The inverted
>    form resulted in migrations to CPUs of slightly lower capacity. Guarded
>    the check for architectural capacity with the sched_cluster_active
>    static key.
>  - Patch 4: Expanded the patch description to explain the behavior of
>    overloaded groups and low-capacity clusters with spare capacity.
>  - Added Reviewed-by tags from Christian. Thanks!
>  - Link to v2: https://lore.kernel.org/r/20260429-rneri-fix-cas-clusters-v2-0-cd787de35cc6@xxxxxxxxxxxxxxx
> 
> Changes in v2:
>  - Patch 1: Rewrote patch description for clarity. Added a note
>    clarifying that SD_ASYM_CPUCAPACITY and SMT are mutually
>    exclusive. (Tim)
>  - Patch 2: Fixed a bug where the capacity check inadvertently broke
>    the mutual exclusion of the sched_reduced_capacity() path. Keep
>    marking the root domain as overloaded when misfit tasks are present
>    to allow bigger CPUs to help via newly idle balance. (sashiko)
>    Fixed the description to state that capacity_greater() looks for
>    differences of ~5% or more, not 20%. (Christian)
>  - Patch 3: Use arch_scale_cpu_capacity() instead of capacity_of() to
>    ignore runtime capacity variability. Inverted the capacity check.
>    (Christian)
>  - Patch 4: Reworded the patch description for clarity.
>  - Link to v1: https://lore.kernel.org/r/20260330-rneri-fix-cas-clusters-v1-0-1e465b6fecb2@xxxxxxxxxxxxxxx/
> 
> ---
> Ricardo Neri (6):
>       sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings
>       sched/fair: Also gate overloaded status update for SD_ASYM_CPUCAPACITY
>       sched/fair: Check CPU capacity before comparing group types during load balance
>       sched/fair: Skip misfit load accounting when the destination CPU cannot help
>       sched/fair: Allow load balancing between CPUs of identical capacity
>       sched/topology: Do not clear SD_PREFER_SIBLING in domains with clusters
> 
>  include/linux/sched/sd_flags.h |  3 +-
>  kernel/sched/fair.c            | 66 ++++++++++++++++++++++++++++++------------
>  kernel/sched/topology.c        | 14 +++++++--
>  3 files changed, 62 insertions(+), 21 deletions(-)
> ---
> base-commit: 50436392fe2359ea108fd27308f86c8283be1622
> change-id: 20250620-rneri-fix-cas-clusters-bb4287d1e152
> 
> Best regards,
> -- 
> Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
>