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

[Chen, Yu C]

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@xxxxxxxxxxxxx>
> > Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > Co-developed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >
> > 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@xxxxxxxxx/
will fix it accordingly. Thanks!

thanks,
Chenyu