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, 94 insertions, 0 deletions

diff --git a/net/ethtool/phc_vclocks.c b/net/ethtool/phc_vclocks.c
new file mode 100644
index 000000000..15146e38a
--- /dev/null
+++ b/net/ethtool/phc_vclocks.c
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2021 NXP
+ */
+#include "common.h"
+#include "netlink.h"
+
+struct phc_vclocks_req_info {
+	struct ethnl_req_info		base;
+};
+
+struct phc_vclocks_reply_data {
+	struct ethnl_reply_data		base;
+	int				num;
+	int				*index;
+};
+
+#define PHC_VCLOCKS_REPDATA(__reply_base) \
+	container_of(__reply_base, struct phc_vclocks_reply_data, base)
+
+const struct nla_policy ethnl_phc_vclocks_get_policy[] = {
+	[ETHTOOL_A_PHC_VCLOCKS_HEADER] = NLA_POLICY_NESTED(ethnl_header_policy),
+};
+
+static int phc_vclocks_prepare_data(const struct ethnl_req_info *req_base,
+				    struct ethnl_reply_data *reply_base,
+				    const struct genl_info *info)
+{
+	struct phc_vclocks_reply_data *data = PHC_VCLOCKS_REPDATA(reply_base);
+	struct net_device *dev = reply_base->dev;
+	int ret;
+
+	ret = ethnl_ops_begin(dev);
+	if (ret < 0)
+		return ret;
+	data->num = ethtool_get_phc_vclocks(dev, &data->index);
+	ethnl_ops_complete(dev);
+
+	return ret;
+}
+
+static int phc_vclocks_reply_size(const struct ethnl_req_info *req_base,
+				  const struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+	int len = 0;
+
+	if (data->num > 0) {
+		len += nla_total_size(sizeof(u32));
+		len += nla_total_size(sizeof(s32) * data->num);
+	}
+
+	return len;
+}
+
+static int phc_vclocks_fill_reply(struct sk_buff *skb,
+				  const struct ethnl_req_info *req_base,
+				  const struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+
+	if (data->num <= 0)
+		return 0;
+
+	if (nla_put_u32(skb, ETHTOOL_A_PHC_VCLOCKS_NUM, data->num) ||
+	    nla_put(skb, ETHTOOL_A_PHC_VCLOCKS_INDEX,
+		    sizeof(s32) * data->num, data->index))
+		return -EMSGSIZE;
+
+	return 0;
+}
+
+static void phc_vclocks_cleanup_data(struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+
+	kfree(data->index);
+}
+
+const struct ethnl_request_ops ethnl_phc_vclocks_request_ops = {
+	.request_cmd		= ETHTOOL_MSG_PHC_VCLOCKS_GET,
+	.reply_cmd		= ETHTOOL_MSG_PHC_VCLOCKS_GET_REPLY,
+	.hdr_attr		= ETHTOOL_A_PHC_VCLOCKS_HEADER,
+	.req_info_size		= sizeof(struct phc_vclocks_req_info),
+	.reply_data_size	= sizeof(struct phc_vclocks_reply_data),
+
+	.prepare_data		= phc_vclocks_prepare_data,
+	.reply_size		= phc_vclocks_reply_size,
+	.fill_reply		= phc_vclocks_fill_reply,
+	.cleanup_data		= phc_vclocks_cleanup_data,
+};