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, 62 insertions, 0 deletions

diff --git a/drivers/md/bcache/stats.h b/drivers/md/bcache/stats.h
new file mode 100644
index 000000000..21b445f8a
--- /dev/null
+++ b/drivers/md/bcache/stats.h
@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _BCACHE_STATS_H_
+#define _BCACHE_STATS_H_
+
+struct cache_stat_collector {
+	atomic_t cache_hits;
+	atomic_t cache_misses;
+	atomic_t cache_bypass_hits;
+	atomic_t cache_bypass_misses;
+	atomic_t cache_miss_collisions;
+	atomic_t sectors_bypassed;
+};
+
+struct cache_stats {
+	struct kobject		kobj;
+
+	unsigned long cache_hits;
+	unsigned long cache_misses;
+	unsigned long cache_bypass_hits;
+	unsigned long cache_bypass_misses;
+	unsigned long cache_miss_collisions;
+	unsigned long sectors_bypassed;
+
+	unsigned int		rescale;
+};
+
+struct cache_accounting {
+	struct closure		cl;
+	struct timer_list	timer;
+	atomic_t		closing;
+
+	struct cache_stat_collector collector;
+
+	struct cache_stats total;
+	struct cache_stats five_minute;
+	struct cache_stats hour;
+	struct cache_stats day;
+};
+
+struct cache_set;
+struct cached_dev;
+struct bcache_device;
+
+void bch_cache_accounting_init(struct cache_accounting *acc,
+			       struct closure *parent);
+
+int bch_cache_accounting_add_kobjs(struct cache_accounting *acc,
+				   struct kobject *parent);
+
+void bch_cache_accounting_clear(struct cache_accounting *acc);
+
+void bch_cache_accounting_destroy(struct cache_accounting *acc);
+
+void bch_mark_cache_accounting(struct cache_set *c, struct bcache_device *d,
+			       bool hit, bool bypass);
+void bch_mark_cache_miss_collision(struct cache_set *c,
+				   struct bcache_device *d);
+void bch_mark_sectors_bypassed(struct cache_set *c,
+			       struct cached_dev *dc,
+			       int sectors);
+
+#endif /* _BCACHE_STATS_H_ */