Re: [PATCH v3 14/21] sched/cache: Respect LLC preference in task migration and detach

["Chen, Yu C" <yu.c.chen@intel.com>]

On 2/18/2026 5:14 PM, Madadi Vineeth Reddy wrote:
> On 11/02/26 03:48, Tim Chen wrote:
>> During the final step of load balancing, can_migrate_task() now
>> considers a task's LLC preference before moving it out of its
>> preferred LLC.
>>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
>> Co-developed-by: Chen Yu <yu.c.chen@intel.com>
>> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
>> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
>> ---
>>
>> Notes:
>>      v2->v3:
>>      Use the similar mechanism as NUMA balancing, which skips over
>>      the tasks that would degrade locality in can_migrate_task();
>>      and only if nr_balanced_failed is high enough do we ignore that.
>>      (Peter Zijlstra)
>>      
>>      Let migrate_degrade_locality() take precedence over
>>      migrate_degrades_llc(), which aims to migrate towards the preferred
>>      NUMA node. (Peter Zijlstra)
>>
>>   kernel/sched/fair.c  | 64 +++++++++++++++++++++++++++++++++++++++++---
>>   kernel/sched/sched.h | 13 +++++++++
>>   2 files changed, 73 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 03959a701514..d1145997b88d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -9973,8 +9973,8 @@ static enum llc_mig can_migrate_llc(int src_cpu, int dst_cpu,
>>    * Check if task p can migrate from source LLC to
>>    * destination LLC in terms of cache aware load balance.
>>    */
>> -static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
>> -							struct task_struct *p)
>> +static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
>> +					 struct task_struct *p)
>>   {
>>   	struct mm_struct *mm;
>>   	bool to_pref;
>> @@ -10041,6 +10041,47 @@ alb_break_llc(struct lb_env *env)
>>   
>>   	return false;
>>   }
>> +
>> +/*
>> + * Check if migrating task p from env->src_cpu to
>> + * env->dst_cpu breaks LLC localiy.
>> + */
>> +static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
>> +{
>> +	if (!sched_cache_enabled())
>> +		return false;
>> +
>> +	if (task_has_sched_core(p))
>> +		return false;
>> +	/*
>> +	 * Skip over tasks that would degrade LLC locality;
>> +	 * only when nr_balanced_failed is sufficiently high do we
>> +	 * ignore this constraint.
>> +	 *
>> +	 * Threshold of cache_nice_tries is set to 1 higher
>> +	 * than nr_balance_failed to avoid excessive task
>> +	 * migration at the same time. Refer to comments around
>> +	 * llc_balance().
>> +	 */
>> +	if (env->sd->nr_balance_failed >= env->sd->cache_nice_tries + 1)
>> +		return false;
>> +
>> +	/*
>> +	 * We know the env->src_cpu has some tasks prefer to
>> +	 * run on env->dst_cpu, skip the tasks do not prefer
>> +	 * env->dst_cpu, and find the one that prefers.
>> +	 */
>> +	if (env->migration_type == migrate_llc_task &&
>> +	    task_llc(p) != llc_id(env->dst_cpu))
>> +		return true;
> 
> `task_llc(p)` returns the LLC id of the CPU the task is currently running on, right?
> Wouldn’t we need to check the task’s *preferred* LLC instead?
> 
> Am I missing something?
> 

No, you did not miss anything; this is indeed a bug.
I realized this during our discussion yesterday:
https://lore.kernel.org/all/22e975fd-4e31-498d-a016-2168721f532a@intel.com/
will fix it accordingly. Thanks!

thanks,
Chenyu