Re: [PATCH v3] sched/cache: Honor asym packing over cache aware scheduling on hybrid systems

[Tim Chen]

On Wed, 2026-10-07 at 10:25 -0700, Tim Chen wrote:
> A regression was reported on an AMD Ryzen AI HX 370 running a cache
> intensive Clang full-LTO link. The little cores run at a much lower
> frequency (3.3 GHz vs 5.1 GHz) and have only half of the L3 cache
> (8 MB vs 16 MB), so pinning such a task to the little-core LLC
> hurts twice, and full-LTO builds slow down dramatically compared to
> pre-cache-aware-scheduling kernels.
> 
> Asym packing and cache aware scheduling express conflicting placement
> strategies. Asym packing wants a task to run on the highest priority CPU,
> whereas cache aware scheduling wants to co-locate the tasks of a process
> on one LLC regardless of the priority of CPUs in that LLC.
> 
> When asym packing tries to migrate task to an idle core that has higher
> priority than source cpu, let asym packing win.  Moving tasks to a higher
> performing idle core will buy more performance than cache co-location.
> 
> Prioritize asym packing over LLC balancing for regular and active load
> balancing.
> 

Hi Peter,

A quick ping on this patch. Do you think it is ready to be applied?

Thanks.

Tim

> Fixes: 23b2b5ccc45c ("sched/cache: Introduce helper functions to enforce LLC migration policy")
> Reported-by: Klaus Kusche <klaus.kusche@xxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/2180ea5a-eb28-4152-8d4d-cd00b0c24b2e@xxxxxxxxxxxxxxx/
> Suggested-by: Kayra Cizmeci <kayracizmeci@xxxxxxxxx>
> Reviewed-by: Kayra Cizmeci <kayracizmeci@xxxxxxxxx>
> Tested-by: Klaus Kusche <klaus.kusche@xxxxxxxxxxxxxxx>
> Tested-by: Ricardo Neri <ricardo.neri@xxxxxxxxx>
> Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 7.2.x
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
> 
> Notes:
>     v3: Remove redundant SD_ASYM_PACKING check in llc_balance() (Kayra Cizmeci).
>     Add Tested-by tag from Chen Yu and Reviewed-by tag from Kayra Cizmeci.
>     Link v2:
>     https://lore.kernel.org/lkml/77ceef1e51b895760dc5f6c9cde985a1679545b5.1791224900.git.tim.c.chen@xxxxxxxxxxxxxxx/
> 
>  kernel/sched/fair.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 57360f5cdde4f..e4906486a4fe9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10934,6 +10934,8 @@ static inline bool task_misfits_asym_cpu(struct lb_env *env, struct task_struct
>  	return false;
>  }
>  
> +static inline bool sched_asym(struct sched_domain *sd, int dst_cpu, int src_cpu);
> +
>  /*
>   * Check if task p can migrate from source LLC to
>   * destination LLC in terms of cache aware load balance.
> @@ -10958,6 +10960,10 @@ static enum llc_mig can_migrate_llc_task(struct lb_env *env,
>  	if (cpu < 0 || cpus_share_cache(src_cpu, dst_cpu))
>  		return mig_unrestricted;
>  
> +	/* Prioritize asym packing to idle core over cache awareness */
> +	if (env->idle && sched_asym(env->sd, dst_cpu, src_cpu))
> +		return mig_unrestricted;
> +
>  	/* skip cache aware load balance for too many threads */
>  	if (invalid_llc_nr(grp, p, dst_cpu) ||
>  	    exceed_llc_capacity(grp, dst_cpu)) {
> @@ -12154,6 +12160,14 @@ static inline bool llc_balance(struct lb_env *env, struct sg_lb_stats *sgs,
>  	    sgs->group_misfit_task_load)
>  		return false;
>  
> +	/*
> +	 * On asym packing domains, if the destination CPU
> +	 * has higher priority than all CPUs in the source group,
> +	 * prioritize asym packing.
> +	 */
> +	if (sgs->group_asym_packing)
> +		return false;
> +
>  	/*
>  	 * Skip cache aware tagging if nr_balanced_failed is sufficiently high.
>  	 * Threshold of cache_nice_tries is set to 1 higher than nr_balance_failed
> @@ -13569,12 +13583,12 @@ static int need_active_balance(struct lb_env *env)
>  {
>  	struct sched_domain *sd = env->sd;
>  
> -	if (alb_break_llc(env))
> -		return 0;
> -
>  	if (asym_active_balance(env))
>  		return 1;
>  
> +	if (alb_break_llc(env))
> +		return 0;
> +
>  	if (imbalanced_active_balance(env))
>  		return 1;
>