Re: [PATCH] sched/cache: Check reduced capacity before alb_break_llc()

[Chen Yu]

Hi Xusheng,

On Thu, Oct 29, 2026 at 10:36:50AM +0800, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> 
> need_active_balance() moves the only CFS task off a CPU whose capacity
> is reduced, for example by IRQ load, to an idle CPU with more.
> alb_break_llc() runs first and refuses to move a lone task off its
> preferred LLC, so if no other CPU in that LLC is idle the task stays on
> the reduced CPU.  Like asym packing, which was moved ahead of
> alb_break_llc() for the same reason, this is a performance case that
> cache locality should not override.
> 
> With two LLCs of two CPUs, the other CPU of the task's LLC busy and 60%
> of the task's CPU spent in a hardirq, a CPU-bound two-thread process
> stayed there for 20 seconds at 38% of its normal throughput.  With this
> change it moves to the idle LLC within a second, as it does with cache
> aware scheduling disabled.
> 
> Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cfc982205b98..e313c6d212c6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13585,12 +13585,6 @@ static int need_active_balance(struct lb_env *env)
>  	if (asym_active_balance(env))
>  		return 1;
>  
> -	if (alb_break_llc(env))
> -		return 0;
> -
> -	if (imbalanced_active_balance(env))
> -		return 1;
> -
>  	/*
>  	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>  	 * It's worth migrating the task if the src_cpu's capacity is reduced
> @@ -13604,6 +13598,12 @@ static int need_active_balance(struct lb_env *env)
>  			return 1;
>  	}
>  
> +	if (alb_break_llc(env))
> +		return 0;
> +
> +	if (imbalanced_active_balance(env))
> +		return 1;
> +
>  	if (env->migration_type == migrate_misfit ||
>  	    env->migration_type == migrate_llc_task)
>  		return 1;
>

Moving the capacity check earlier seems to completely ignore the
task's LLC preference. It seems to be a trade-off between choosing
CPU capacity and cache re-population on a remote LLC, and cross-LLC
access latency due to cross-LLC migration. I don't know which one has
more impact, it looks to be workload-specific. May I know which
benchmark/platform you used to find this issue? I'd like to launch
hackbench/tbench/schbenh/netperf(or yours) to evaluate the impact.

May I know if the issue is because of the
if (env->src_rq->nr_running <= 1)
	return true
in alb_break_llc()?

If yes, to mitigate the possible impact, do you think tuning the
alb_break_llc() would be safer?

thanks,
Chenyu

patch1 introduces a helper to consider the capacity between src_cpu
and dst_cpu, no functional change:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0f2364479f9d..35e6cc7a0fb4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13573,10 +13573,27 @@ imbalanced_active_balance(struct lb_env *env)
 	return 0;
 }
 
-static int need_active_balance(struct lb_env *env)
+static inline bool
+reduced_cap_alb(struct lb_env *env)
 {
 	struct sched_domain *sd = env->sd;
 
+	/*
+	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
+	 * It's worth migrating the task if the src_cpu's capacity is reduced
+	 * because of other sched_class or IRQs if more capacity stays
+	 * available on dst_cpu.
+	 */
+	return env->idle &&
+	       (env->src_rq->cfs.h_nr_runnable == 1) &&
+	       check_cpu_capacity(env->src_rq, sd) &&
+	       (capacity_of(env->src_cpu)*sd->imbalance_pct < capacity_of(env->dst_cpu)*100);
+}
+
+static int need_active_balance(struct lb_env *env)
+{
+	bool enough_cap_src = !reduced_cap_alb(env);
+
 	if (alb_break_llc(env))
 		return 0;
 
@@ -13586,18 +13603,8 @@ static int need_active_balance(struct lb_env *env)
 	if (imbalanced_active_balance(env))
 		return 1;
 
-	/*
-	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
-	 * It's worth migrating the task if the src_cpu's capacity is reduced
-	 * because of other sched_class or IRQs if more capacity stays
-	 * available on dst_cpu.
-	 */
-	if (env->idle &&
-	    (env->src_rq->cfs.h_nr_runnable == 1)) {
-		if ((check_cpu_capacity(env->src_rq, sd)) &&
-		    (capacity_of(env->src_cpu)*sd->imbalance_pct < capacity_of(env->dst_cpu)*100))
-			return 1;
-	}
+	if (!enough_cap_src)
+		return 1;
 
 	if (env->migration_type == migrate_misfit ||
 	    env->migration_type == migrate_llc_task)



patch2 tunes the abl_break_llc():

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 35e6cc7a0fb4..43ce929f8ad1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10994,7 +10994,7 @@ static enum llc_mig can_migrate_llc_task(struct lb_env *env,
  * migrating the only runnable task away.
  */
 static inline bool
-alb_break_llc(struct lb_env *env)
+alb_break_llc(struct lb_env *env, bool enough_cap_src)
 {
 	if (!sched_cache_enabled())
 		return false;
@@ -11022,7 +11022,7 @@ alb_break_llc(struct lb_env *env)
 		if (env->migration_type == migrate_misfit)
 			return false;
 
-		if (env->src_rq->nr_running <= 1)
+		if (env->src_rq->nr_running <= 1 && enough_cap_src)
 			return true;
 
 		cur = rcu_dereference_all(env->src_rq->curr);
@@ -11098,7 +11098,7 @@ static inline bool get_llc_stats(int cpu, unsigned long *util,
 }
 
 static inline bool
-alb_break_llc(struct lb_env *env)
+alb_break_llc(struct lb_env *env, bool enough_cap_src)
 {
 	return false;
 }
@@ -13594,7 +13594,7 @@ static int need_active_balance(struct lb_env *env)
 {
 	bool enough_cap_src = !reduced_cap_alb(env);
 
-	if (alb_break_llc(env))
+	if (alb_break_llc(env, enough_cap_src))
 		return 0;
 
 	if (asym_active_balance(env))