[RFC PATCH v3 16/17] sched/cache: Drop stale preferred LLC checks

[Jianyong Wu]

With multi-LLC aggregation, an mm's preferred LLC can be
outside a task's preferred NUMA node. Do not discard that LLC when
calculating the task's preference.

Likewise, an LLC migration may select a destination other than the
task's preferred LLC. Let can_migrate_llc_task() decide whether the
migration is allowed instead of rejecting the task based on an exact
preferred-LLC match.

Signed-off-by: Jianyong Wu <wujianyong@xxxxxxxx>
---
 kernel/sched/fair.c | 29 +----------------------------
 1 file changed, 1 insertion(+), 28 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b93b6e6c9dea..5d11f9b7f685 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1669,27 +1669,9 @@ static int get_pref_llc(struct task_struct *p, struct mm_struct *mm)
 		return -1;
 
 	mm_sched_cpu = READ_ONCE(mm->sc_stat.cpu);
-	if (mm_sched_cpu != -1) {
+	if (mm_sched_cpu != -1)
 		mm_sched_llc = llc_id(mm_sched_cpu);
 
-#ifdef CONFIG_NUMA_BALANCING
-		/*
-		 * Don't assign preferred LLC if it
-		 * conflicts with NUMA balancing.
-		 * This can happen when sched_setnuma() gets
-		 * called, however it is not much of an issue
-		 * because we expect account_mm_sched() to get
-		 * called fairly regularly -- at a higher rate
-		 * than sched_setnuma() at least -- and thus the
-		 * conflict only exists for a short period of time.
-		 */
-		if (static_branch_likely(&sched_numa_balancing) &&
-		    p->numa_preferred_nid >= 0 &&
-		    cpu_to_node(mm_sched_cpu) != p->numa_preferred_nid)
-			mm_sched_llc = -1;
-#endif
-	}
-
 	return mm_sched_llc;
 }
 
@@ -10898,15 +10880,6 @@ static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
 	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 &&
-	    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)
 		return false;
-- 
2.34.1