[Patch v4 16/16] sched/cache: Fix stale preferred_llc for a new task

[Tim Chen]

From: Chen Yu <yu.c.chen@xxxxxxxxx>

On fork without CLONE_VM, the child gets a new mm,
the parent's preferred_llc value is stale for the
child.

Fix this by resetting the task's preferred_llc to -1.

This bug was reported by sashiko.

Fixes: 47d8696b95f7 ("sched/cache: Assign preferred LLC ID to processes")
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Co-developed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7e64cd18727e..73da6f8fc9ec 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1914,6 +1914,11 @@ void init_sched_mm(struct task_struct *p)
 
 	init_task_work(work, task_cache_work);
 	work->next = work;
+	/*
+	 * Reset new task's preference to avoid
+	 * polluting account_llc_enqueue().
+	 */
+	p->preferred_llc = -1;
 }
 
 #else /* CONFIG_SCHED_CACHE */
-- 
2.32.0