sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

alb_break_llc() decides whether to break LLC preference during active
load balance. It does so by testing that every runnable fair task on the
source rq prefers its LLC:

	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable

But the two counters cover different sets. nr_pref_llc_running is updated
in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
clear_delayed() and drops delay-dequeued tasks.

So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
alb_break_llc() returns false, and active balance is free to pull a task
off its preferred LLC. Active balance only moves runnable tasks, and this
is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
skips the per-task test in can_migrate_task(). The runnable set is the one
we want.

Fix it on the counter side. A task should be counted in
nr_pref_llc_running exactly while it is both queued on its preferred LLC
(pref_llc_queued) and runnable (!sched_delayed). Define that membership
once in task_pref_llc_runnable(), and adjust the counter only through
pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
change either input: account_llc_enqueue(), account_llc_dequeue(),
set_delayed() and clear_delayed(). Gating every update on the same
predicate keeps the delay, wake and dequeue paths from double-counting
or underflowing; see the comments at those sites for the ordering.

nr_llc_running and sd->llc_counts are not touched and stay on queued
semantics.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 66 insertions, 0 deletions

diff --git a/net/wireless/tests/util.h b/net/wireless/tests/util.h
new file mode 100644
index 000000000..6de712e0d
--- /dev/null
+++ b/net/wireless/tests/util.h
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Utilities for cfg80211 unit testing
+ *
+ * Copyright (C) 2023 Intel Corporation
+ */
+#ifndef __CFG80211_UTILS_H
+#define __CFG80211_UTILS_H
+
+#define CHAN2G(_freq)  { \
+	.band = NL80211_BAND_2GHZ, \
+	.center_freq = (_freq), \
+	.hw_value = (_freq), \
+}
+
+static const struct ieee80211_channel channels_2ghz[] = {
+	CHAN2G(2412), /* Channel 1 */
+	CHAN2G(2417), /* Channel 2 */
+	CHAN2G(2422), /* Channel 3 */
+	CHAN2G(2427), /* Channel 4 */
+	CHAN2G(2432), /* Channel 5 */
+	CHAN2G(2437), /* Channel 6 */
+	CHAN2G(2442), /* Channel 7 */
+	CHAN2G(2447), /* Channel 8 */
+	CHAN2G(2452), /* Channel 9 */
+	CHAN2G(2457), /* Channel 10 */
+	CHAN2G(2462), /* Channel 11 */
+	CHAN2G(2467), /* Channel 12 */
+	CHAN2G(2472), /* Channel 13 */
+	CHAN2G(2484), /* Channel 14 */
+};
+
+struct t_wiphy_priv {
+	struct kunit *test;
+	struct cfg80211_ops *ops;
+
+	void *ctx;
+
+	struct ieee80211_supported_band band_2ghz;
+	struct ieee80211_channel channels_2ghz[ARRAY_SIZE(channels_2ghz)];
+};
+
+#define T_WIPHY(test, ctx) ({						\
+		struct wiphy *__wiphy =					\
+			kunit_alloc_resource(test, t_wiphy_init,	\
+					     t_wiphy_exit,		\
+					     GFP_KERNEL, &(ctx));	\
+									\
+		KUNIT_ASSERT_NOT_NULL(test, __wiphy);			\
+		__wiphy;						\
+	})
+#define t_wiphy_ctx(wiphy) (((struct t_wiphy_priv *)wiphy_priv(wiphy))->ctx)
+
+int t_wiphy_init(struct kunit_resource *resource, void *data);
+void t_wiphy_exit(struct kunit_resource *resource);
+
+#define t_skb_remove_member(skb, type, member)	do {				\
+		memmove((skb)->data + (skb)->len - sizeof(type) +		\
+			offsetof(type, member),					\
+			(skb)->data + (skb)->len - sizeof(type) +		\
+			offsetofend(type, member),				\
+			offsetofend(type, member));				\
+		skb_trim(skb, (skb)->len - sizeof_field(type, member));		\
+	} while (0)
+
+#endif /* __CFG80211_UTILS_H */