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, 72 insertions, 0 deletions

diff --git a/scripts/gcc-plugins/Makefile b/scripts/gcc-plugins/Makefile
new file mode 100644
index 000000000..05b14aba4
--- /dev/null
+++ b/scripts/gcc-plugins/Makefile
@@ -0,0 +1,72 @@
+# SPDX-License-Identifier: GPL-2.0
+
+$(obj)/randomize_layout_plugin.so: $(obj)/randomize_layout_seed.h
+quiet_cmd_create_randomize_layout_seed = SEEDHDR $@
+cmd_create_randomize_layout_seed = \
+	SEED=$$(cat $(filter-out FORCE,$^) </dev/null); \
+	echo '/*' > $@; \
+	echo ' * This file is automatically generated. Keep it private.' >> $@; \
+	echo ' * Exposing this value will expose the layout of randomized structures.' >> $@; \
+	echo ' */' >> $@; \
+	echo "const char *randstruct_seed = \"$$SEED\";" >> $@
+$(obj)/randomize_layout_seed.h: $(objtree)/scripts/basic/randstruct.seed FORCE
+	$(call if_changed,create_randomize_layout_seed)
+targets += randomize_layout_seed.h
+
+# Build rules for plugins
+#
+# No extra code is needed for single-file plugins.
+# For multi-file plugins, use *-objs syntax to list the objects.
+#
+# If the plugin foo.so is compiled from foo.c and foo2.c, you can do:
+#
+# foo-objs := foo.o foo2.o
+
+always-y += $(GCC_PLUGIN)
+
+GCC_PLUGINS_DIR = $(shell $(CC) -print-file-name=plugin)
+
+plugin_cxxflags	= -Wp,-MMD,$(depfile) $(KBUILD_HOSTCXXFLAGS) -fPIC \
+		  -include $(srctree)/include/linux/compiler-version.h \
+		  -DPLUGIN_VERSION=$(call stringify,$(KERNELVERSION)) \
+		  -I $(GCC_PLUGINS_DIR)/include -I $(obj) \
+		  -fno-rtti -fno-exceptions -fasynchronous-unwind-tables \
+		  -ggdb -Wno-narrowing -Wno-unused-variable \
+		  -Wno-format-diag
+
+plugin_ldflags	= -shared
+
+plugin-single	:= $(foreach m, $(GCC_PLUGIN), $(if $($(m:%.so=%-objs)),,$(m)))
+plugin-multi	:= $(filter-out $(plugin-single), $(GCC_PLUGIN))
+plugin-objs	:= $(sort $(foreach m, $(plugin-multi), $($(m:%.so=%-objs))))
+
+targets += $(plugin-single) $(plugin-multi) $(plugin-objs)
+clean-files += *.so
+
+plugin-single	:= $(addprefix $(obj)/, $(plugin-single))
+plugin-multi	:= $(addprefix $(obj)/, $(plugin-multi))
+plugin-objs	:= $(addprefix $(obj)/, $(plugin-objs))
+
+quiet_cmd_plugin_cxx_so_c = HOSTCXX $@
+      cmd_plugin_cxx_so_c = $(HOSTCXX) $(plugin_cxxflags) $(plugin_ldflags) -o $@ $<
+
+$(plugin-single): $(obj)/%.so: $(src)/%.c FORCE
+	$(call if_changed_dep,plugin_cxx_so_c)
+
+quiet_cmd_plugin_ld_so_o = HOSTLD  $@
+      cmd_plugin_ld_so_o = $(HOSTCXX) $(plugin_ldflags) -o $@ \
+			   $(addprefix $(obj)/, $($(target-stem)-objs))
+
+$(plugin-multi): FORCE
+	$(call if_changed,plugin_ld_so_o)
+$(foreach m, $(notdir $(plugin-multi)), $(eval $(obj)/$m: $(addprefix $(obj)/, $($(m:%.so=%-objs)))))
+
+quiet_cmd_plugin_cxx_o_c = HOSTCXX $@
+      cmd_plugin_cxx_o_c = $(HOSTCXX) $(plugin_cxxflags) -c -o $@ $<
+
+$(plugin-objs): $(obj)/%.o: $(src)/%.c FORCE
+	$(call if_changed_dep,plugin_cxx_o_c)
+
+$(obj)/../../include/generated/gcc-plugins.h: $(plugin-single) $(plugin-multi) FORCE
+	$(call if_changed,touch)
+always-y += ../../include/generated/gcc-plugins.h