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, 88 insertions, 0 deletions

diff --git a/Documentation/networking/vxlan.rst b/Documentation/networking/vxlan.rst
new file mode 100644
index 000000000..2759dc1cc
--- /dev/null
+++ b/Documentation/networking/vxlan.rst
@@ -0,0 +1,88 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================================
+Virtual eXtensible Local Area Networking documentation
+======================================================
+
+The VXLAN protocol is a tunnelling protocol designed to solve the
+problem of limited VLAN IDs (4096) in IEEE 802.1q.  With VXLAN the
+size of the identifier is expanded to 24 bits (16777216).
+
+VXLAN is described by IETF RFC 7348, and has been implemented by a
+number of vendors.  The protocol runs over UDP using a single
+destination port.  This document describes the Linux kernel tunnel
+device, there is also a separate implementation of VXLAN for
+Openvswitch.
+
+Unlike most tunnels, a VXLAN is a 1 to N network, not just point to
+point. A VXLAN device can learn the IP address of the other endpoint
+either dynamically in a manner similar to a learning bridge, or make
+use of statically-configured forwarding entries.
+
+The management of vxlan is done in a manner similar to its two closest
+neighbors GRE and VLAN. Configuring VXLAN requires the version of
+iproute2 that matches the kernel release where VXLAN was first merged
+upstream.
+
+1. Create vxlan device::
+
+    # ip link add vxlan0 type vxlan id 42 group 239.1.1.1 dev eth1 dstport 4789
+
+This creates a new device named vxlan0.  The device uses the multicast
+group 239.1.1.1 over eth1 to handle traffic for which there is no
+entry in the forwarding table.  The destination port number is set to
+the IANA-assigned value of 4789.  The Linux implementation of VXLAN
+pre-dates the IANA's selection of a standard destination port number
+and uses the Linux-selected value by default to maintain backwards
+compatibility.
+
+2. Delete vxlan device::
+
+    # ip link delete vxlan0
+
+3. Show vxlan info::
+
+    # ip -d link show vxlan0
+
+It is possible to create, destroy and display the vxlan
+forwarding table using the new bridge command.
+
+1. Create forwarding table entry::
+
+    # bridge fdb add to 00:17:42:8a:b4:05 dst 192.19.0.2 dev vxlan0
+
+2. Delete forwarding table entry::
+
+    # bridge fdb delete 00:17:42:8a:b4:05 dev vxlan0
+
+3. Show forwarding table::
+
+    # bridge fdb show dev vxlan0
+
+The following NIC features may indicate support for UDP tunnel-related
+offloads (most commonly VXLAN features, but support for a particular
+encapsulation protocol is NIC specific):
+
+ - `tx-udp_tnl-segmentation`
+ - `tx-udp_tnl-csum-segmentation`
+    ability to perform TCP segmentation offload of UDP encapsulated frames
+
+ - `rx-udp_tunnel-port-offload`
+    receive side parsing of UDP encapsulated frames which allows NICs to
+    perform protocol-aware offloads, like checksum validation offload of
+    inner frames (only needed by NICs without protocol-agnostic offloads)
+
+For devices supporting `rx-udp_tunnel-port-offload` the list of currently
+offloaded ports can be interrogated with `ethtool`::
+
+  $ ethtool --show-tunnels eth0
+  Tunnel information for eth0:
+    UDP port table 0:
+      Size: 4
+      Types: vxlan
+      No entries
+    UDP port table 1:
+      Size: 4
+      Types: geneve, vxlan-gpe
+      Entries (1):
+          port 1230, vxlan-gpe