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, 63 insertions, 0 deletions

diff --git a/include/net/llc_if.h b/include/net/llc_if.h
new file mode 100644
index 000000000..a33325961
--- /dev/null
+++ b/include/net/llc_if.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LLC_IF_H
+#define LLC_IF_H
+/*
+ * Copyright (c) 1997 by Procom Technology,Inc.
+ * 		 2001 by Arnaldo Carvalho de Melo <acme@conectiva.com.br>
+ */
+/* Defines LLC interface to network layer */
+/* Available primitives */
+#include <linux/if.h>
+#include <linux/if_arp.h>
+#include <linux/llc.h>
+#include <linux/etherdevice.h>
+#include <net/llc.h>
+
+#define LLC_DATAUNIT_PRIM	1
+#define LLC_CONN_PRIM		2
+#define LLC_DATA_PRIM		3
+#define LLC_DISC_PRIM		4
+#define LLC_RESET_PRIM		5
+#define LLC_FLOWCONTROL_PRIM	6 /* Not supported at this time */
+#define LLC_DISABLE_PRIM	7
+#define LLC_XID_PRIM		8
+#define LLC_TEST_PRIM		9
+#define LLC_SAP_ACTIVATION     10
+#define LLC_SAP_DEACTIVATION   11
+
+#define LLC_NBR_PRIMITIVES     11
+
+#define LLC_IND			1
+#define LLC_CONFIRM		2
+
+/* Primitive type */
+#define LLC_PRIM_TYPE_REQ	1
+#define LLC_PRIM_TYPE_IND	2
+#define LLC_PRIM_TYPE_RESP	3
+#define LLC_PRIM_TYPE_CONFIRM	4
+
+/* Reset reasons, remote entity or local LLC */
+#define LLC_RESET_REASON_REMOTE	1
+#define LLC_RESET_REASON_LOCAL	2
+
+/* Disconnect reasons */
+#define LLC_DISC_REASON_RX_DM_RSP_PDU	0
+#define LLC_DISC_REASON_RX_DISC_CMD_PDU	1
+#define LLC_DISC_REASON_ACK_TMR_EXP	2
+
+/* Confirm reasons */
+#define LLC_STATUS_CONN		0 /* connect confirm & reset confirm */
+#define LLC_STATUS_DISC		1 /* connect confirm & reset confirm */
+#define LLC_STATUS_FAILED	2 /* connect confirm & reset confirm */
+#define LLC_STATUS_IMPOSSIBLE	3 /* connect confirm */
+#define LLC_STATUS_RECEIVED	4 /* data conn */
+#define LLC_STATUS_REMOTE_BUSY	5 /* data conn */
+#define LLC_STATUS_REFUSE	6 /* data conn */
+#define LLC_STATUS_CONFLICT	7 /* disconnect conn */
+#define LLC_STATUS_RESET_DONE	8 /*  */
+
+int llc_establish_connection(struct sock *sk, const u8 *lmac, u8 *dmac,
+			     u8 dsap);
+int llc_build_and_send_pkt(struct sock *sk, struct sk_buff *skb);
+int llc_send_disc(struct sock *sk);
+#endif /* LLC_IF_H */