Re: [PATCH v2 1/3] sched/fair: Add smt_balance and llc_balance to the decision matrix

[Tim Chen]

On Fri, 2026-10-02 at 23:46 +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. Against a
> local has_spare group it forces the pull when prefer_sibling is set,
> because the group_llc_balance test comes before sibling_imbalance() in
> sched_balance_find_src_group(). SD_PREFER_SIBLING is only cleared for
> NUMA domains. Against a local fully_busy or imbalanced group the nr_idle
> checks apply, and against a local overloaded group the local group is
> busier and the pair is balanced.
> 
> No functional change.
> 
> Signed-off-by: Jemmy Wong <jemmywong512@xxxxxxxxx>

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Tim

> ---
>  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 57360f5cdde4..bcb9987952b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13184,13 +13184,15 @@ 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
> + * llc_balance      force     nr_idle    N/A   N/A  N/A  nr_idle   N/A  balanced
> + * overloaded       force     force      N/A   N/A  N/A  force     N/A  avg_load
>   *
>   * N/A :      Not Applicable because already filtered while updating
>   *            statistics.