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, 180 insertions, 0 deletions

diff --git a/net/sunrpc/sysctl.c b/net/sunrpc/sysctl.c
new file mode 100644
index 000000000..bdb587a72
--- /dev/null
+++ b/net/sunrpc/sysctl.c
@@ -0,0 +1,180 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * linux/net/sunrpc/sysctl.c
+ *
+ * Sysctl interface to sunrpc module.
+ *
+ * I would prefer to register the sunrpc table below sys/net, but that's
+ * impossible at the moment.
+ */
+
+#include <linux/types.h>
+#include <linux/linkage.h>
+#include <linux/ctype.h>
+#include <linux/fs.h>
+#include <linux/sysctl.h>
+#include <linux/module.h>
+
+#include <linux/uaccess.h>
+#include <linux/sunrpc/types.h>
+#include <linux/sunrpc/sched.h>
+#include <linux/sunrpc/stats.h>
+#include <linux/sunrpc/svc_xprt.h>
+
+#include "netns.h"
+
+/*
+ * Declare the debug flags here
+ */
+unsigned int	rpc_debug;
+EXPORT_SYMBOL_GPL(rpc_debug);
+
+unsigned int	nfs_debug;
+EXPORT_SYMBOL_GPL(nfs_debug);
+
+unsigned int	nfsd_debug;
+EXPORT_SYMBOL_GPL(nfsd_debug);
+
+unsigned int	nlm_debug;
+EXPORT_SYMBOL_GPL(nlm_debug);
+
+#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
+
+static int proc_do_xprt(const struct ctl_table *table, int write,
+			void *buffer, size_t *lenp, loff_t *ppos)
+{
+	char tmpbuf[256];
+	ssize_t len;
+
+	if (write || *ppos) {
+		*lenp = 0;
+		return 0;
+	}
+	len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
+	len = memory_read_from_buffer(buffer, *lenp, ppos, tmpbuf, len);
+
+	if (len < 0) {
+		*lenp = 0;
+		return -EINVAL;
+	}
+	*lenp = len;
+	return 0;
+}
+
+static int
+proc_dodebug(const struct ctl_table *table, int write, void *buffer, size_t *lenp,
+	     loff_t *ppos)
+{
+	char		tmpbuf[20], *s = NULL;
+	char *p;
+	unsigned int	value;
+	size_t		left, len;
+
+	if ((*ppos && !write) || !*lenp) {
+		*lenp = 0;
+		return 0;
+	}
+
+	left = *lenp;
+
+	if (write) {
+		p = buffer;
+		while (left && isspace(*p)) {
+			left--;
+			p++;
+		}
+		if (!left)
+			goto done;
+
+		if (left > sizeof(tmpbuf) - 1)
+			return -EINVAL;
+		memcpy(tmpbuf, p, left);
+		tmpbuf[left] = '\0';
+
+		value = simple_strtol(tmpbuf, &s, 0);
+		if (s) {
+			left -= (s - tmpbuf);
+			if (left && !isspace(*s))
+				return -EINVAL;
+			while (left && isspace(*s)) {
+				left--;
+				s++;
+			}
+		} else
+			left = 0;
+		*(unsigned int *) table->data = value;
+		/* Display the RPC tasks on writing to rpc_debug */
+		if (strcmp(table->procname, "rpc_debug") == 0)
+			rpc_show_tasks(&init_net);
+	} else {
+		len = sprintf(tmpbuf, "0x%04x", *(unsigned int *) table->data);
+		if (len > left)
+			len = left;
+		memcpy(buffer, tmpbuf, len);
+		if ((left -= len) > 0) {
+			*((char *)buffer + len) = '\n';
+			left--;
+		}
+	}
+
+done:
+	*lenp -= left;
+	*ppos += *lenp;
+	return 0;
+}
+
+static struct ctl_table_header *sunrpc_table_header;
+
+static struct ctl_table debug_table[] = {
+	{
+		.procname	= "rpc_debug",
+		.data		= &rpc_debug,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dodebug
+	},
+	{
+		.procname	= "nfs_debug",
+		.data		= &nfs_debug,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dodebug
+	},
+	{
+		.procname	= "nfsd_debug",
+		.data		= &nfsd_debug,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dodebug
+	},
+	{
+		.procname	= "nlm_debug",
+		.data		= &nlm_debug,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dodebug
+	},
+	{
+		.procname	= "transports",
+		.maxlen		= 256,
+		.mode		= 0444,
+		.proc_handler	= proc_do_xprt,
+	},
+};
+
+void
+rpc_register_sysctl(void)
+{
+	if (!sunrpc_table_header)
+		sunrpc_table_header = register_sysctl("sunrpc", debug_table);
+}
+
+void
+rpc_unregister_sysctl(void)
+{
+	if (sunrpc_table_header) {
+		unregister_sysctl_table(sunrpc_table_header);
+		sunrpc_table_header = NULL;
+	}
+}
+#endif