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, 51 insertions, 0 deletions

diff --git a/net/atm/common.h b/net/atm/common.h
new file mode 100644
index 000000000..11cb51dd7
--- /dev/null
+++ b/net/atm/common.h
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* net/atm/common.h - ATM sockets (common part for PVC and SVC) */
+
+/* Written 1995-2000 by Werner Almesberger, EPFL LRC/ICA */
+
+
+#ifndef NET_ATM_COMMON_H
+#define NET_ATM_COMMON_H
+
+#include <linux/net.h>
+#include <linux/poll.h> /* for poll_table */
+
+
+int vcc_create(struct net *net, struct socket *sock, int protocol, int family, int kern);
+int vcc_release(struct socket *sock);
+int vcc_connect(struct socket *sock, int itf, short vpi, int vci);
+int vcc_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
+		int flags);
+int vcc_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len);
+__poll_t vcc_poll(struct file *file, struct socket *sock, poll_table *wait);
+int vcc_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg);
+int vcc_compat_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg);
+int vcc_setsockopt(struct socket *sock, int level, int optname,
+		   sockptr_t optval, unsigned int optlen);
+int vcc_getsockopt(struct socket *sock, int level, int optname,
+		   sockopt_t *opt);
+void vcc_process_recv_queue(struct atm_vcc *vcc);
+
+int atmpvc_init(void);
+void atmpvc_exit(void);
+int atm_sysfs_init(void);
+void atm_sysfs_exit(void);
+
+#ifdef CONFIG_PROC_FS
+int atm_proc_init(void);
+void atm_proc_exit(void);
+#else
+static inline int atm_proc_init(void)
+{
+	return 0;
+}
+
+static inline void atm_proc_exit(void)
+{
+	/* nothing */
+}
+#endif /* CONFIG_PROC_FS */
+
+void atm_dev_release_vccs(struct atm_dev *dev);
+
+#endif