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, 324 insertions, 0 deletions

diff --git a/net/sched/sch_codel.c b/net/sched/sch_codel.c
new file mode 100644
index 000000000..6aa5829d6
--- /dev/null
+++ b/net/sched/sch_codel.c
@@ -0,0 +1,324 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+/*
+ * Codel - The Controlled-Delay Active Queue Management algorithm
+ *
+ *  Copyright (C) 2011-2012 Kathleen Nichols <nichols@pollere.com>
+ *  Copyright (C) 2011-2012 Van Jacobson <van@pollere.net>
+ *
+ *  Implemented on linux by :
+ *  Copyright (C) 2012 Michael D. Taht <dave.taht@bufferbloat.net>
+ *  Copyright (C) 2012,2015 Eric Dumazet <edumazet@google.com>
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/skbuff.h>
+#include <linux/prefetch.h>
+#include <net/pkt_sched.h>
+#include <net/codel.h>
+#include <net/codel_impl.h>
+#include <net/codel_qdisc.h>
+
+
+#define DEFAULT_CODEL_LIMIT 1000
+
+struct codel_sched_data {
+	struct codel_params	params;
+	struct codel_vars	vars;
+	struct codel_stats	stats;
+	u32			drop_overlimit;
+};
+
+/* This is the specific function called from codel_dequeue()
+ * to dequeue a packet from queue. Note: backlog is handled in
+ * codel, we dont need to reduce it here.
+ */
+static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
+{
+	struct Qdisc *sch = ctx;
+	struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
+
+	if (skb) {
+		qstats_backlog_sub(sch, qdisc_pkt_len(skb));
+		prefetch(&skb->end); /* we'll need skb_shinfo() */
+	}
+	return skb;
+}
+
+static void drop_func(struct sk_buff *skb, void *ctx)
+{
+	struct Qdisc *sch = ctx;
+
+	qdisc_dequeue_drop(sch, skb, QDISC_DROP_CONGESTED);
+	qdisc_qstats_drop(sch);
+}
+
+static struct sk_buff *__codel_qdisc_dequeue(struct Qdisc *sch)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+	struct sk_buff *skb;
+
+	skb = codel_dequeue(sch, &sch->qstats.backlog, &q->params, &q->vars,
+			    &q->stats, qdisc_pkt_len, codel_get_enqueue_time,
+			    drop_func, dequeue_func);
+
+	if (skb)
+		qdisc_bstats_update(sch, skb);
+	return skb;
+}
+
+static void codel_dequeue_drop(struct Qdisc *sch)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+
+	if (q->stats.drop_count) {
+		qdisc_tree_reduce_backlog(sch, q->stats.drop_count,
+					  q->stats.drop_len);
+		q->stats.drop_count = 0;
+		q->stats.drop_len = 0;
+	}
+}
+
+static struct sk_buff *codel_qdisc_dequeue(struct Qdisc *sch)
+{
+	struct sk_buff *skb;
+
+	skb = __codel_qdisc_dequeue(sch);
+
+	codel_dequeue_drop(sch);
+
+	return skb;
+}
+
+static struct sk_buff *codel_peek(struct Qdisc *sch)
+{
+	struct sk_buff *skb = skb_peek(&sch->gso_skb);
+
+	if (!skb) {
+		skb = __codel_qdisc_dequeue(sch);
+
+		if (skb) {
+			__skb_queue_head(&sch->gso_skb, skb);
+			/* it's still part of the queue */
+			qdisc_qstats_backlog_inc(sch, skb);
+			sch->q.qlen++;
+		}
+
+		codel_dequeue_drop(sch);
+	}
+
+	return skb;
+}
+
+static int codel_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+			       struct sk_buff **to_free)
+{
+	struct codel_sched_data *q;
+
+	if (likely(qdisc_qlen(sch) < sch->limit)) {
+		codel_set_enqueue_time(skb);
+		return qdisc_enqueue_tail(skb, sch);
+	}
+	q = qdisc_priv(sch);
+	WRITE_ONCE(q->drop_overlimit, q->drop_overlimit + 1);
+	return qdisc_drop_reason(skb, sch, to_free, QDISC_DROP_OVERLIMIT);
+}
+
+static const struct nla_policy codel_policy[TCA_CODEL_MAX + 1] = {
+	[TCA_CODEL_TARGET]	= { .type = NLA_U32 },
+	[TCA_CODEL_LIMIT]	= { .type = NLA_U32 },
+	[TCA_CODEL_INTERVAL]	= { .type = NLA_U32 },
+	[TCA_CODEL_ECN]		= { .type = NLA_U32 },
+	[TCA_CODEL_CE_THRESHOLD]= { .type = NLA_U32 },
+};
+
+static int codel_change(struct Qdisc *sch, struct nlattr *opt,
+			struct netlink_ext_ack *extack)
+{
+	unsigned int dropped_pkts = 0, dropped_bytes = 0;
+	struct codel_sched_data *q = qdisc_priv(sch);
+	struct nlattr *tb[TCA_CODEL_MAX + 1];
+	int err;
+
+	err = nla_parse_nested_deprecated(tb, TCA_CODEL_MAX, opt,
+					  codel_policy, NULL);
+	if (err < 0)
+		return err;
+
+	sch_tree_lock(sch);
+
+	if (tb[TCA_CODEL_TARGET]) {
+		u32 target = nla_get_u32(tb[TCA_CODEL_TARGET]);
+
+		WRITE_ONCE(q->params.target,
+			   ((u64)target * NSEC_PER_USEC) >> CODEL_SHIFT);
+	}
+
+	if (tb[TCA_CODEL_CE_THRESHOLD]) {
+		u64 val = nla_get_u32(tb[TCA_CODEL_CE_THRESHOLD]);
+
+		WRITE_ONCE(q->params.ce_threshold,
+			   (val * NSEC_PER_USEC) >> CODEL_SHIFT);
+	}
+
+	if (tb[TCA_CODEL_INTERVAL]) {
+		u32 interval = nla_get_u32(tb[TCA_CODEL_INTERVAL]);
+
+		WRITE_ONCE(q->params.interval,
+			   ((u64)interval * NSEC_PER_USEC) >> CODEL_SHIFT);
+	}
+
+	if (tb[TCA_CODEL_LIMIT])
+		WRITE_ONCE(sch->limit,
+			   nla_get_u32(tb[TCA_CODEL_LIMIT]));
+
+	if (tb[TCA_CODEL_ECN])
+		WRITE_ONCE(q->params.ecn,
+			   !!nla_get_u32(tb[TCA_CODEL_ECN]));
+
+	while (sch->q.qlen > sch->limit) {
+		struct sk_buff *skb = qdisc_dequeue_internal(sch, true);
+
+		if (!skb)
+			break;
+
+		dropped_pkts++;
+		dropped_bytes += qdisc_pkt_len(skb);
+		rtnl_qdisc_drop(skb, sch);
+	}
+	qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes);
+
+	sch_tree_unlock(sch);
+	return 0;
+}
+
+static int codel_init(struct Qdisc *sch, struct nlattr *opt,
+		      struct netlink_ext_ack *extack)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+
+	sch->limit = DEFAULT_CODEL_LIMIT;
+
+	codel_params_init(&q->params);
+	codel_vars_init(&q->vars);
+	codel_stats_init(&q->stats);
+	q->params.mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 256, 1 << 20);
+
+	if (opt) {
+		int err = codel_change(sch, opt, extack);
+
+		if (err)
+			return err;
+	}
+
+	if (sch->limit >= 1)
+		sch->flags |= TCQ_F_CAN_BYPASS;
+	else
+		sch->flags &= ~TCQ_F_CAN_BYPASS;
+
+	sch->flags |= TCQ_F_DEQUEUE_DROPS;
+
+	return 0;
+}
+
+static int codel_dump(struct Qdisc *sch, struct sk_buff *skb)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+	codel_time_t ce_threshold;
+	struct nlattr *opts;
+
+	opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
+	if (opts == NULL)
+		goto nla_put_failure;
+
+	if (nla_put_u32(skb, TCA_CODEL_TARGET,
+			codel_time_to_us(READ_ONCE(q->params.target))) ||
+	    nla_put_u32(skb, TCA_CODEL_LIMIT,
+			READ_ONCE(sch->limit)) ||
+	    nla_put_u32(skb, TCA_CODEL_INTERVAL,
+			codel_time_to_us(READ_ONCE(q->params.interval))) ||
+	    nla_put_u32(skb, TCA_CODEL_ECN,
+			READ_ONCE(q->params.ecn)))
+		goto nla_put_failure;
+	ce_threshold = READ_ONCE(q->params.ce_threshold);
+	if (ce_threshold != CODEL_DISABLED_THRESHOLD &&
+	    nla_put_u32(skb, TCA_CODEL_CE_THRESHOLD,
+			codel_time_to_us(ce_threshold)))
+		goto nla_put_failure;
+	return nla_nest_end(skb, opts);
+
+nla_put_failure:
+	nla_nest_cancel(skb, opts);
+	return -1;
+}
+
+static int codel_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
+{
+	const struct codel_sched_data *q = qdisc_priv(sch);
+	struct tc_codel_xstats st = {
+		.maxpacket	= READ_ONCE(q->stats.maxpacket),
+		.count		= READ_ONCE(q->vars.count),
+		.lastcount	= READ_ONCE(q->vars.lastcount),
+		.drop_overlimit = READ_ONCE(q->drop_overlimit),
+		.ldelay		= codel_time_to_us(READ_ONCE(q->vars.ldelay)),
+		.dropping	= READ_ONCE(q->vars.dropping),
+		.ecn_mark	= READ_ONCE(q->stats.ecn_mark),
+		.ce_mark	= READ_ONCE(q->stats.ce_mark),
+	};
+
+	if (st.dropping) {
+		codel_tdiff_t delta = READ_ONCE(q->vars.drop_next) - codel_get_time();
+
+		if (delta >= 0)
+			st.drop_next = codel_time_to_us(delta);
+		else
+			st.drop_next = -codel_time_to_us(-delta);
+	}
+
+	return gnet_stats_copy_app(d, &st, sizeof(st));
+}
+
+static void codel_reset(struct Qdisc *sch)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+
+	qdisc_reset_queue(sch);
+	codel_vars_init(&q->vars);
+}
+
+static struct Qdisc_ops codel_qdisc_ops __read_mostly = {
+	.id		=	"codel",
+	.priv_size	=	sizeof(struct codel_sched_data),
+
+	.enqueue	=	codel_qdisc_enqueue,
+	.dequeue	=	codel_qdisc_dequeue,
+	.peek		=	codel_peek,
+	.init		=	codel_init,
+	.reset		=	codel_reset,
+	.change 	=	codel_change,
+	.dump		=	codel_dump,
+	.dump_stats	=	codel_dump_stats,
+	.owner		=	THIS_MODULE,
+};
+MODULE_ALIAS_NET_SCH("codel");
+
+static int __init codel_module_init(void)
+{
+	return register_qdisc(&codel_qdisc_ops);
+}
+
+static void __exit codel_module_exit(void)
+{
+	unregister_qdisc(&codel_qdisc_ops);
+}
+
+module_init(codel_module_init)
+module_exit(codel_module_exit)
+
+MODULE_DESCRIPTION("Controlled Delay queue discipline");
+MODULE_AUTHOR("Dave Taht");
+MODULE_AUTHOR("Eric Dumazet");
+MODULE_LICENSE("Dual BSD/GPL");