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, 65 insertions, 0 deletions

diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
new file mode 100644
index 000000000..f71775090
--- /dev/null
+++ b/net/mptcp/fastopen.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+/* MPTCP Fast Open Mechanism
+ *
+ * Copyright (c) 2021-2022, Dmytro SHYTYI
+ */
+
+#include "protocol.h"
+
+void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subflow,
+					      struct request_sock *req)
+{
+	struct sock *sk, *ssk;
+	struct sk_buff *skb;
+	struct tcp_sock *tp;
+	bool has_rxtstamp;
+
+	/* on early fallback the subflow context is deleted by
+	 * subflow_syn_recv_sock()
+	 */
+	if (!subflow)
+		return;
+
+	ssk = subflow->tcp_sock;
+	sk = subflow->conn;
+	tp = tcp_sk(ssk);
+
+	/* A valid TFO cookie does not guarantee SYN data. */
+	skb = skb_peek(&ssk->sk_receive_queue);
+	if (!skb)
+		return;
+
+	subflow->is_mptfo = 1;
+
+	/* dequeue the skb from sk receive queue */
+	__skb_unlink(skb, &ssk->sk_receive_queue);
+	skb_ext_reset(skb);
+
+	mptcp_subflow_lend_fwdmem(subflow, skb);
+
+	/* We copy the fastopen data, but that don't belong to the mptcp sequence
+	 * space, need to offset it in the subflow sequence, see mptcp_subflow_get_map_offset()
+	 */
+	tp->copied_seq += skb->len;
+	subflow->ssn_offset += skb->len;
+	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
+
+	/* Only the sequence delta is relevant */
+	MPTCP_SKB_CB(skb)->map_seq = -skb->len;
+	MPTCP_SKB_CB(skb)->end_seq = 0;
+	MPTCP_SKB_CB(skb)->offset = 0;
+	MPTCP_SKB_CB(skb)->has_rxtstamp = has_rxtstamp;
+	MPTCP_SKB_CB(skb)->cant_coalesce = 1;
+
+	mptcp_data_lock(sk);
+	DEBUG_NET_WARN_ON_ONCE(sock_owned_by_user_nocheck(sk));
+
+	mptcp_borrow_fwdmem(sk, skb);
+	skb_set_owner_r(skb, sk);
+	__skb_queue_tail(&sk->sk_receive_queue, skb);
+	mptcp_sk(sk)->bytes_received += skb->len;
+
+	sk->sk_data_ready(sk);
+
+	mptcp_data_unlock(sk);
+}