Re: [PATCH 1/3] sched/fair: Add smt_balance and llc_balance to the decision matrix

[Tim Chen]

On Mon, 2026-09-28 at 12:20 +0800, Jemmy Wong wrote:
> The group-type matrix was introduced in commit 0b0695f2b34a
> ("sched/fair: Rework load_balance()"). When commit fee1759e4f04
> ("sched/fair: Determine active load balance for SMT sched groups")
> added group_smt_balance and commit f38cc2f0d8a3 ("sched/cache:
> Prioritize tasks preferring destination LLC during balancing")
> added group_llc_balance, neither commit updated the matrix table.
> 
> Both types are only tagged on non-local groups in update_sg_lb_stats(),
> so their local columns are N/A.
> 
> As busiest, group_smt_balance is only set when dst_cpu is idle and the
> SMT group runs more than one task. Against a local has_spare or
> fully_busy group it goes through the nr_idle checks, where a non-SMT
> dst group may also force the pull via smt_vs_nonsmt_groups(). Against a
> local imbalanced or overloaded group the local group is busier and the
> pair is balanced.
> 
> As busiest, group_llc_balance is not an unconditional force. A local
> overloaded group is busier and the pair is balanced; otherwise the
> nr_idle checks apply, with prefer_sibling still able to force the pull
> when the local group has spare capacity.
> 
> No functional change.
> 
> Signed-off-by: Jemmy Wong <jemmywong512@xxxxxxxxx>
> ---
>  kernel/sched/fair.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7455a83a6a99..f9ddcecfd19d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12947,13 +12947,15 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  /*
>   * Decision matrix according to the local and busiest group type:
>   *
> - * busiest \ local has_spare fully_busy misfit asym imbalanced overloaded
> - * has_spare        nr_idle   balanced   N/A    N/A  balanced   balanced
> - * fully_busy       nr_idle   nr_idle    N/A    N/A  balanced   balanced
> - * misfit_task      force     N/A        N/A    N/A  N/A        N/A
> - * asym_packing     force     force      N/A    N/A  force      force
> - * imbalanced       force     force      N/A    N/A  force      force
> - * overloaded       force     force      N/A    N/A  force      avg_load
> + * busiest \ local has_spare fully_busy misfit smt asym imbalanced llc overloaded
> + * has_spare        nr_idle   balanced   N/A   N/A  N/A  balanced  N/A  balanced
> + * fully_busy       nr_idle   nr_idle    N/A   N/A  N/A  balanced  N/A  balanced
> + * misfit_task      force     N/A        N/A   N/A  N/A  N/A       N/A  N/A
> + * smt_balance      nr_idle   nr_idle    N/A   N/A  N/A  balanced  N/A  balanced
> + * asym_packing     force     force      N/A   N/A  N/A  force     N/A  force
> + * imbalanced       force     force      N/A   N/A  N/A  force     N/A  force

Thanks for picking this up - nice to have the table match the code
again. I walked the new rows and columns against
sched_balance_find_src_group(), and they line up, with one exception:

> + * llc_balance      nr_idle   nr_idle    N/A   N/A  N/A  nr_idle   N/A  balanced

I think local=has_spare, busiest=llc_balance is "force", not "nr_idle":

        if (sds.prefer_sibling && local->group_type == group_has_spare &&
            (busiest->group_type == group_llc_balance ||
             sibling_imbalance(env, &sds, busiest, local) > 1))
                goto force_balance;

The group_llc_balance clause short-circuits the sibling_imbalance()
test, so this pair forces unconditionally once prefer_sibling is set,
and prefer_sibling is set for LLC-vs-LLC balancing: the groups span
per-LLC (SD_SHARE_LLC) domains, which keep SD_PREFER_SIBLING (only
SD_NUMA strips it). That also matches your changelog ("prefer_sibling
still able to force the pull...") - the table just reads nr_idle where
the prose says force.

Could you flip that one cell?

 * llc_balance      force     nr_idle    N/A   N/A  N/A  nr_idle   N/A  balanced

Thanks.

Tim

> + * overloaded       force     force      N/A   N/A  N/A  force     N/A  avg_load
>   *
>   * N/A :      Not Applicable because already filtered while updating
>   *            statistics.