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, 85 insertions, 0 deletions

diff --git a/net/bridge/Kconfig b/net/bridge/Kconfig
new file mode 100644
index 000000000..318715c8f
--- /dev/null
+++ b/net/bridge/Kconfig
@@ -0,0 +1,85 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# 802.1d Ethernet Bridging
+#
+
+config BRIDGE
+	tristate "802.1d Ethernet Bridging"
+	select LLC
+	select STP
+	help
+	  If you say Y here, then your Linux box will be able to act as an
+	  Ethernet bridge, which means that the different Ethernet segments it
+	  is connected to will appear as one Ethernet to the participants.
+	  Several such bridges can work together to create even larger
+	  networks of Ethernets using the IEEE 802.1 spanning tree algorithm.
+	  As this is a standard, Linux bridges will cooperate properly with
+	  other third party bridge products.
+
+	  In order to use the Ethernet bridge, you'll need the bridge
+	  configuration tools; see <file:Documentation/networking/bridge.rst>
+	  for location. Please read the Bridge mini-HOWTO for more
+	  information.
+
+	  If you enable iptables support along with the bridge support then you
+	  turn your bridge into a bridging IP firewall.
+	  iptables will then see the IP packets being bridged, so you need to
+	  take this into account when setting up your firewall rules.
+	  Enabling arptables support when bridging will let arptables see
+	  bridged ARP traffic in the arptables FORWARD chain.
+
+	  To compile this code as a module, choose M here: the module
+	  will be called bridge.
+
+	  If unsure, say N.
+
+config BRIDGE_IGMP_SNOOPING
+	bool "IGMP/MLD snooping"
+	depends on BRIDGE
+	depends on INET
+	default y
+	help
+	  If you say Y here, then the Ethernet bridge will be able selectively
+	  forward multicast traffic based on IGMP/MLD traffic received from
+	  each port.
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say Y.
+
+config BRIDGE_VLAN_FILTERING
+	bool "VLAN filtering"
+	depends on BRIDGE
+	depends on VLAN_8021Q
+	default n
+	help
+	  If you say Y here, then the Ethernet bridge will be able selectively
+	  receive and forward traffic based on VLAN information in the packet
+	  any VLAN information configured on the bridge port or bridge device.
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say Y.
+
+config BRIDGE_MRP
+	bool "MRP protocol"
+	depends on BRIDGE
+	default n
+	help
+	  If you say Y here, then the Ethernet bridge will be able to run MRP
+	  protocol to detect loops
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say N.
+
+config BRIDGE_CFM
+	bool "CFM protocol"
+	depends on BRIDGE
+	help
+	  If you say Y here, then the Ethernet bridge will be able to run CFM
+	  protocol according to 802.1Q section 12.14
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say N.