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

[Kayra Cizmeci]

Helloooo Tim,

> 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
> strategy. 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 empty
> 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.


> +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.
> @@ -10847,6 +10849,10 @@ 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 over cache awareness */
> +	if (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)) {
> @@ -12043,6 +12049,15 @@ 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 ((env->sd->flags & SD_ASYM_PACKING) &&
> +	    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
> @@ -13458,12 +13473,12 @@ 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;
 
Hope I could test this. But I don't really have hardware for it :-(.

Regardless tho.

Let's say when entering can_migrate_llc_task(), dst_cpu is CPU0, while src_cpu is CPU1.
And CPU0 has a bigger asym_prio than CPU1. No SMT. When entering can_migrate_llc_task() and sched_asym()
from there sched_use_asym_prio() returns true without checking if the core is fully idle or not.
And sched_asym_prefer() comes back true too, so sched_asym() returns true and, we just returned mig_unrestricted.

I could be missing something, If I'm not tho is that on purpose? If it is, the last paragraph needs to change
since it says that "asym packing tries to migrate task to an empty core" and after that "higher performing idle core"

Thanks,
Kayra