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, 52 insertions, 0 deletions

diff --git a/io_uring/memmap.h b/io_uring/memmap.h
new file mode 100644
index 000000000..f4cfbb6b9
--- /dev/null
+++ b/io_uring/memmap.h
@@ -0,0 +1,52 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef IO_URING_MEMMAP_H
+#define IO_URING_MEMMAP_H
+
+#define IORING_MAP_OFF_PARAM_REGION		0x20000000ULL
+#define IORING_MAP_OFF_ZCRX_REGION		0x30000000ULL
+
+#define IORING_OFF_ZCRX_SHIFT		16
+
+struct page **io_pin_pages(unsigned long uaddr, unsigned long len, int *npages);
+
+#ifndef CONFIG_MMU
+unsigned int io_uring_nommu_mmap_capabilities(struct file *file);
+#endif
+unsigned long io_uring_get_unmapped_area(struct file *file, unsigned long addr,
+					 unsigned long len, unsigned long pgoff,
+					 unsigned long flags);
+int io_uring_mmap(struct file *file, struct vm_area_struct *vma);
+
+void io_free_region(struct user_struct *user, struct io_mapped_region *mr);
+int io_create_region(struct io_ring_ctx *ctx, struct io_mapped_region *mr,
+		     struct io_uring_region_desc *reg,
+		     unsigned long mmap_offset);
+
+static inline void *io_region_get_ptr(struct io_mapped_region *mr)
+{
+	return mr->ptr;
+}
+
+static inline bool io_region_is_set(struct io_mapped_region *mr)
+{
+	return !!mr->nr_pages;
+}
+
+static inline void io_region_publish(struct io_ring_ctx *ctx,
+				     struct io_mapped_region *src_region,
+				     struct io_mapped_region *dst_region)
+{
+	/*
+	 * Once published mmap can find it without holding only the ->mmap_lock
+	 * and not ->uring_lock.
+	 */
+	guard(mutex)(&ctx->mmap_lock);
+	*dst_region = *src_region;
+}
+
+static inline size_t io_region_size(struct io_mapped_region *mr)
+{
+	return (size_t) mr->nr_pages << PAGE_SHIFT;
+}
+
+#endif