[PATCH] sched/cache: Fix a thread aggregation conflict when there is one runnable task

[PATCH] sched/cache: Fix a thread aggregation conflict when there is one runnable task

[Chen Yu]

On systems with SD_ASYM_PACKING set at the PKG domain level (e.g.,
with ITMT enabled), the sched_asym() check in sched_balance_find_src_rq()
prevents pulling a single task from a higher-priority CPU to a
lower-priority CPU. This blocks migrate_llc_task migrations where a task
wants to move to its preferred LLC for cache locality.

For example, CPU 0 (highest priority) has a task preferring LLC3,
but the load balancer on CPU 120 (LLC3) cannot pull it because:
sched_asym(PKG_sd, cpu=0, dst_cpu=120) and nr_running == 1
The task remains stuck in the wrong LLC indefinitely.

Fix this by exempting migrate_llc_task from the sched_asym filter.

Fixes: e4c9a4cb244a ("sched/cache: Add migrate_llc_task migration type for cache-aware balancing")
Reported-by: Yi Lai <yi1.lai@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..e6e6fa84c9e1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12986,8 +12986,12 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 *
 		 * If balancing between cores, let lower priority CPUs help
 		 * SMT cores with more than one busy sibling.
+		 *
+		 * For migrate_llc_task, skip this check: cache locality
+		 * outweighs the asym priority.
 		 */
-		if (sched_asym(env->sd, i, env->dst_cpu) && nr_running == 1)
+		if (sched_asym(env->sd, i, env->dst_cpu) && nr_running == 1 &&
+		    env->migration_type != migrate_llc_task)
 			continue;
 
 		switch (env->migration_type) {
-- 
2.25.1

Re: [PATCH] sched/cache: Fix a thread aggregation conflict when there is one runnable task

[Zhan Xusheng]

On Mon, Jul 27, 2026 at 09:29:14AM +0800, Chen Yu wrote:
> -	if (sched_asym(env->sd, i, env->dst_cpu) && nr_running == 1)
> +	if (sched_asym(env->sd, i, env->dst_cpu) && nr_running == 1 &&
> +	    env->migration_type != migrate_llc_task)
>  		continue;

The fix looks correct for the ITMT case. I also convinced myself it does
not introduce a migrate_llc_task <-> asym-packing ping-pong: once the task
lands in its preferred LLC, a subsequent asym pull-back is a migrate_task,
which goes through can_migrate_task() -> migrate_degrades_llc() ->
can_migrate_llc_task() == mig_forbid (moving away from the preferred LLC),
so it returns 0 and the task is not pulled back (modulo the
nr_balance_failed >= cache_nice_tries + 1 escape). So it should not
oscillate. Nice.

One minor consistency question: the adjacent single-task filter just above,

	if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
	    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
	    nr_running == 1)
		continue;

has the same nr_running == 1 shape and is not exempted for
migrate_llc_task. I don't think it is reachable on current hardware (it
would need both multiple LLCs, so migrate_llc_task can fire, and
asymmetric CPU capacity at that domain), so this is not a correctness
concern for the reported case. But since the two filters now treat
migrate_llc_task differently, was leaving the capacity one intentional
(capacity prioritized over cache locality on asym-capacity), or just out
of scope here? A word in the changelog would make the intent clear.

Thanks,
Xusheng