Re: [PATCH] sched/fair: avoid creating misfits during cache-aware balancing

[Tim Chen]

On Tue, 2026-08-25 at 10:41 -0700, Tim Chen wrote:
> Cache-aware load balancing biases tasks toward their preferred LLC. On
> asymmetric CPU capacity systems (e.g. big.LITTLE) the destination LLC may
> contain CPUs that are too small to run the task. Pulling the task there
> turns it into a misfit, trading a cache-locality gain for a capacity loss
> that's more detrimental to performance.
> 
> Guard both cache-aware migration entry points against this:
> 
>  - can_migrate_llc_task(): forbid the LLC migration when the task fits its
>    source CPU but would not fit the destination CPU.
>  - alb_break_llc(): veto the active balance under the same condition so the
>    runnable task is not pushed onto a CPU that cannot accommodate it.
> 
> Both checks are gated with checks for hybrid processors, so symmetric
> systems are unaffected. Tasks that already do not fit their source CPU
> are left to the existing LLC policy, since the move cannot make their
> fitness worse (this also preserves misfit up-migration to bigger CPUs).
> 
> Additionally, if there are misfit tasks found in the load balancing
> classification phase, prioritize misfit task migrations
> over LLC load aggregation on asymmetric systems. A better fitting
> CPU will boost performance more than better cache locality.
> 
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Tested-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

Forgot my signed off

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

Tim
> ---
>  kernel/sched/fair.c | 50 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 45 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6d881e530f89..cf5c022bbd55 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10691,17 +10691,40 @@ static enum llc_mig can_migrate_llc(int src_cpu, int dst_cpu,
>  	return mig_llc;
>  }
>  
> +static inline bool task_misfits_asym_cpu(struct lb_env *env, struct task_struct *p)
> +{
> +	/*
> +	 * On asymmetric CPU capacity domains, do not let cache-aware
> +	 * balancing pull the task onto a destination CPU that cannot
> +	 * accommodate it. Doing so would turn the task into a misfit on
> +	 * the destination, trading a cache-locality gain for a capacity
> +	 * loss. If the task already does not fit its source CPU, the move
> +	 * cannot make things worse, so let the LLC preference decide.
> +	 */
> +	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) && p &&
> +	    !task_fits_cpu(p, env->dst_cpu) &&
> +	    task_fits_cpu(p, env->src_cpu))
> +		return true;
> +
> +	return false;
> +}
> +
>  /*
>   * Check if task p can migrate from source LLC to
>   * destination LLC in terms of cache aware load balance.
>   */
> -static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
> +static enum llc_mig can_migrate_llc_task(struct lb_env *env,
>  					 struct task_struct *p)
>  {
>  	struct mm_struct *mm;
>  	bool to_pref;
> -	int cpu;
> +	int cpu, src_cpu, dst_cpu;
> +
> +	if (task_misfits_asym_cpu(env, p))
> +		return mig_forbid;
>  
> +	src_cpu = env->src_cpu;
> +	dst_cpu = env->dst_cpu;
>  	mm = p->mm;
>  	if (!mm)
>  		return mig_unrestricted;
> @@ -10758,6 +10781,14 @@ alb_break_llc(struct lb_env *env)
>  		unsigned long util = 0;
>  		struct task_struct *cur;
>  
> +		/*
> +		 * Migrating misfit tasks from current CPU
> +		 * to CPU with a better fit.
> +		 * Prioritize that over LLC preference.
> +		 */
> +		if (env->migration_type == migrate_misfit)
> +			return false;
> +
>  		if (env->src_rq->nr_running <= 1)
>  			return true;
>  
> @@ -10765,7 +10796,8 @@ alb_break_llc(struct lb_env *env)
>  		if (cur && cur->sched_class == &fair_sched_class)
>  			util = task_util(cur);
>  
> -		if (can_migrate_llc(env->src_cpu, env->dst_cpu,
> +		if (task_misfits_asym_cpu(env, cur) ||
> +		    can_migrate_llc(env->src_cpu, env->dst_cpu,
>  				    util, false) == mig_forbid)
>  			return true;
>  	}
> @@ -10805,8 +10837,7 @@ static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
>  	    READ_ONCE(p->preferred_llc) != llc_id(env->dst_cpu))
>  		return true;
>  
> -	if (can_migrate_llc_task(env->src_cpu,
> -				 env->dst_cpu, p) != mig_forbid)
> +	if (can_migrate_llc_task(env, p) != mig_forbid)
>  		return false;
>  
>  	return true;
> @@ -11869,6 +11900,15 @@ static inline bool llc_balance(struct lb_env *env, struct sg_lb_stats *sgs,
>  	if (env->sd->flags & SD_SHARE_LLC)
>  		return false;
>  
> +	/*
> +	 * On asymmetric domains, group_misfit_task_load
> +	 * should be prioritized to move tasks to CPU that fit them
> +	 * over aggregating tasks to their preferred LLC.
> +	 */
> +	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
> +	    sgs->group_misfit_task_load)
> +		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