sched/cache: Fix checking active load balance by only considering the CFS task

The currently running task cur may not be a CFS task, such as
an RT or Deadline task. For non-CFS tasks, the task_util(cur)
utilization average is not maintained, so this might pass a
stale or meaningless value to can_migrate_llc().

Check if the task is CFS before getting its task_util().

This bug was reported by sashiko.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/f9161133cf040d286dca11344a112c5ef2a5253d.1778703694.git.tim.c.chen@linux.intel.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 96c61ce366c2cf8cb56fa481ad9520ab1757569c..c249caea3862099da6a401263038cfb5ab97fcf6 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10509,7 +10509,8 @@ alb_break_llc(struct lb_env *env)
        /*
         * All tasks prefer to stay on their current CPU.
         * Do not pull a task from its preferred CPU if:
-        * 1. It is the only task running there(not too imbalance); OR
+        * 1. It is the only task running and does not exceed
+        *    imbalance allowance; OR
         * 2. Migrating it away from its preferred LLC would violate
         *    the cache-aware scheduling policy.
         */
@@ -10522,7 +10523,7 @@ alb_break_llc(struct lb_env *env)
                        return true;
 
                cur = rcu_dereference_all(env->src_rq->curr);
-               if (cur)
+               if (cur && cur->sched_class == &fair_sched_class)
                        util = task_util(cur);
 
                if (can_migrate_llc(env->src_cpu, env->dst_cpu,