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, 63 insertions, 0 deletions

diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
new file mode 100644
index 000000000..6ead00ec7
--- /dev/null
+++ b/scripts/Makefile.clean
@@ -0,0 +1,63 @@
+# SPDX-License-Identifier: GPL-2.0
+# ==========================================================================
+# Cleaning up
+# ==========================================================================
+
+src := $(srcroot)/$(obj)
+
+PHONY := __clean
+__clean:
+
+include $(srctree)/scripts/Kbuild.include
+include $(kbuild-file)
+
+# Figure out what we need to build from the various variables
+# ==========================================================================
+
+subdir-ymn := $(sort $(subdir-y) $(subdir-m) $(subdir-) \
+		$(patsubst %/,%, $(filter %/, $(obj-y) $(obj-m) $(obj-))))
+
+# Add subdir path
+
+subdir-ymn	:= $(addprefix $(obj)/,$(subdir-ymn))
+
+# build a list of files to remove, usually relative to the current
+# directory
+
+__clean-files	:= \
+	$(clean-files) $(targets) $(hostprogs) $(userprogs) \
+	$(extra-y) $(extra-m) $(extra-) \
+	$(always-y) $(always-m) $(always-) \
+	$(hostprogs-always-y) $(hostprogs-always-m) $(hostprogs-always-) \
+	$(userprogs-always-y) $(userprogs-always-m) $(userprogs-always-)
+
+__clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
+
+__clean-files   := $(wildcard $(addprefix $(obj)/, $(__clean-files)))
+
+# ==========================================================================
+
+# To make this rule robust against "Argument list too long" error,
+# remove $(obj)/ prefix, and restore it by a shell command.
+quiet_cmd_clean = CLEAN   $(obj)
+      cmd_clean = printf '$(obj)/%s ' $(patsubst $(obj)/%,%,$(__clean-files)) | xargs rm -rf
+
+__clean: $(subdir-ymn)
+ifneq ($(strip $(__clean-files)),)
+	$(call cmd,clean)
+endif
+	@:
+
+
+# ===========================================================================
+# Generic stuff
+# ===========================================================================
+
+# Descending
+# ---------------------------------------------------------------------------
+
+PHONY += $(subdir-ymn)
+$(subdir-ymn):
+	$(Q)$(MAKE) $(clean)=$@
+
+.PHONY: $(PHONY)