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, 48 insertions, 0 deletions

diff --git a/mm/vmalloc.h b/mm/vmalloc.h
new file mode 100644
index 000000000..8866ddcff
--- /dev/null
+++ b/mm/vmalloc.h
@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * mm-internal APIs for vmalloc
+ */
+#ifndef __MM_VMALLOC_H
+#define __MM_VMALLOC_H
+
+#include <linux/vmalloc.h>
+
+#ifdef CONFIG_MMU
+void __init vmalloc_init(void);
+int __must_check vmap_pages_range_noflush(unsigned long addr, unsigned long end,
+		pgprot_t prot, struct page **pages,
+		unsigned int page_shift, gfp_t gfp_mask);
+unsigned int get_vm_area_page_order(struct vm_struct *vm);
+#else
+static inline void vmalloc_init(void) {}
+
+static inline
+int __must_check vmap_pages_range_noflush(unsigned long addr, unsigned long end,
+		pgprot_t prot, struct page **pages,
+		unsigned int page_shift, gfp_t gfp_mask)
+{
+	return -EINVAL;
+}
+
+static inline void vunmap_range_noflush(unsigned long start, unsigned long end)
+{
+}
+#endif
+
+struct vm_struct *__get_vm_area_node(unsigned long size,
+		unsigned long align, unsigned long shift,
+		unsigned long vm_flags, unsigned long start,
+		unsigned long end, int node, gfp_t gfp_mask,
+		const void *caller);
+
+void clear_vm_uninitialized_flag(struct vm_struct *vm);
+
+int __must_check __vmap_pages_range_noflush(unsigned long addr,
+		unsigned long end, pgprot_t prot,
+		struct page **pages, unsigned int page_shift);
+
+void vunmap_range_noflush(unsigned long start, unsigned long end);
+
+void __vunmap_range_noflush(unsigned long start, unsigned long end);
+
+#endif /* __MM_VMALLOC_H */