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, 61 insertions, 0 deletions

diff --git a/arch/mips/mm/sc-debugfs.c b/arch/mips/mm/sc-debugfs.c
new file mode 100644
index 000000000..80ff39471
--- /dev/null
+++ b/arch/mips/mm/sc-debugfs.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Paul Burton <paul.burton@mips.com>
+ */
+
+#include <asm/bcache.h>
+#include <asm/debug.h>
+#include <linux/uaccess.h>
+#include <linux/debugfs.h>
+#include <linux/init.h>
+
+static ssize_t sc_prefetch_read(struct file *file, char __user *user_buf,
+				size_t count, loff_t *ppos)
+{
+	bool enabled = bc_prefetch_is_enabled();
+	char buf[3];
+
+	buf[0] = enabled ? 'Y' : 'N';
+	buf[1] = '\n';
+	buf[2] = 0;
+
+	return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
+}
+
+static ssize_t sc_prefetch_write(struct file *file,
+				 const char __user *user_buf,
+				 size_t count, loff_t *ppos)
+{
+	bool enabled;
+	int err;
+
+	err = kstrtobool_from_user(user_buf, count, &enabled);
+	if (err)
+		return err;
+
+	if (enabled)
+		bc_prefetch_enable();
+	else
+		bc_prefetch_disable();
+
+	return count;
+}
+
+static const struct file_operations sc_prefetch_fops = {
+	.open = simple_open,
+	.llseek = default_llseek,
+	.read = sc_prefetch_read,
+	.write = sc_prefetch_write,
+};
+
+static int __init sc_debugfs_init(void)
+{
+	struct dentry *dir;
+
+	dir = debugfs_create_dir("l2cache", mips_debugfs_dir);
+	debugfs_create_file("prefetch", S_IRUGO | S_IWUSR, dir, NULL,
+			    &sc_prefetch_fops);
+	return 0;
+}
+late_initcall(sc_debugfs_init);