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, 151 insertions, 0 deletions

diff --git a/mm/cma_sysfs.c b/mm/cma_sysfs.c
new file mode 100644
index 000000000..d5bf792c6
--- /dev/null
+++ b/mm/cma_sysfs.c
@@ -0,0 +1,151 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * CMA SysFS Interface
+ *
+ * Copyright (c) 2021 Minchan Kim <minchan@kernel.org>
+ */
+
+#include <linux/cma.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+
+#include "cma.h"
+
+#define CMA_ATTR_RO(_name) \
+	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
+
+void cma_sysfs_account_success_pages(struct cma *cma, unsigned long nr_pages)
+{
+	atomic64_add(nr_pages, &cma->nr_pages_succeeded);
+}
+
+void cma_sysfs_account_fail_pages(struct cma *cma, unsigned long nr_pages)
+{
+	atomic64_add(nr_pages, &cma->nr_pages_failed);
+}
+
+void cma_sysfs_account_release_pages(struct cma *cma, unsigned long nr_pages)
+{
+	atomic64_add(nr_pages, &cma->nr_pages_released);
+}
+
+static inline struct cma *cma_from_kobj(struct kobject *kobj)
+{
+	return container_of(kobj, struct cma_kobject, kobj)->cma;
+}
+
+static ssize_t alloc_pages_success_show(struct kobject *kobj,
+					struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%llu\n",
+			  atomic64_read(&cma->nr_pages_succeeded));
+}
+CMA_ATTR_RO(alloc_pages_success);
+
+static ssize_t alloc_pages_fail_show(struct kobject *kobj,
+				     struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%llu\n", atomic64_read(&cma->nr_pages_failed));
+}
+CMA_ATTR_RO(alloc_pages_fail);
+
+static ssize_t release_pages_success_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%llu\n", atomic64_read(&cma->nr_pages_released));
+}
+CMA_ATTR_RO(release_pages_success);
+
+static ssize_t total_pages_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%lu\n", cma->count);
+}
+CMA_ATTR_RO(total_pages);
+
+static ssize_t available_pages_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%lu\n", cma->available_count);
+}
+CMA_ATTR_RO(available_pages);
+
+static void cma_kobj_release(struct kobject *kobj)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+	struct cma_kobject *cma_kobj = cma->cma_kobj;
+
+	kfree(cma_kobj);
+	cma->cma_kobj = NULL;
+}
+
+static struct attribute *cma_attrs[] = {
+	&alloc_pages_success_attr.attr,
+	&alloc_pages_fail_attr.attr,
+	&release_pages_success_attr.attr,
+	&total_pages_attr.attr,
+	&available_pages_attr.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(cma);
+
+static const struct kobj_type cma_ktype = {
+	.release = cma_kobj_release,
+	.sysfs_ops = &kobj_sysfs_ops,
+	.default_groups = cma_groups,
+};
+
+static int __init cma_sysfs_init(void)
+{
+	struct kobject *cma_kobj_root;
+	struct cma_kobject *cma_kobj;
+	struct cma *cma;
+	int i, err;
+
+	cma_kobj_root = kobject_create_and_add("cma", mm_kobj);
+	if (!cma_kobj_root)
+		return -ENOMEM;
+
+	for (i = 0; i < cma_area_count; i++) {
+		cma = &cma_areas[i];
+		if (!test_bit(CMA_ACTIVATED, &cma->flags))
+			continue;
+
+		cma_kobj = kzalloc_obj(*cma_kobj);
+		if (!cma_kobj) {
+			err = -ENOMEM;
+			goto out;
+		}
+
+		cma->cma_kobj = cma_kobj;
+		cma_kobj->cma = cma;
+		err = kobject_init_and_add(&cma_kobj->kobj, &cma_ktype,
+					   cma_kobj_root, "%s", cma->name);
+		if (err) {
+			kobject_put(&cma_kobj->kobj);
+			goto out;
+		}
+	}
+
+	return 0;
+out:
+	while (--i >= 0) {
+		cma = &cma_areas[i];
+		if (cma->cma_kobj)
+			kobject_put(&cma->cma_kobj->kobj);
+	}
+	kobject_put(cma_kobj_root);
+
+	return err;
+}
+subsys_initcall(cma_sysfs_init);