sched/cache: Fix unpaired account_llc_enqueue/dequeue

There is a race condition that, after a task is enqueued
on a runqueue, task_llc(p) may change due to CPU hotplug,
because the llc_id is dynamically allocated and adjusted
at runtime.
Therefore, checking task_llc(p) to determine whether the
task is being dequeued from its preferred LLC is unreliable
and can cause inconsistent values.

To fix this problem, record whether p is enqueued on its
preferred LLC, in order to pair with account_llc_dequeue()
to maintain a consistent nr_pref_llc_running per runqueue.

This bug was reported by sashiko, and the solution was once
suggested by Prateek.

Fixes: 46afe3af7ead ("sched/cache: Track LLC-preferred tasks per runqueue")
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
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/0c8c6a1571d66792a4d2ff0103ba3cc13e059046.1778703694.git.tim.c.chen@linux.intel.com

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 95729670929c..2c9e8e2edde1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1410,6 +1410,8 @@ struct task_struct {
 #ifdef CONFIG_SCHED_CACHE
 	struct callback_head		cache_work;
 	int				preferred_llc;
+	/* 1: task was enqueued to its preferred LLC, 0 otherwise */
+	int				pref_llc_queued;
 #endif
 
 	struct rseq_data		rseq;