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, 98 insertions, 0 deletions

diff --git a/tools/docs/features-refresh.sh b/tools/docs/features-refresh.sh
new file mode 100755
index 000000000..c2288124e
--- /dev/null
+++ b/tools/docs/features-refresh.sh
@@ -0,0 +1,98 @@
+#
+# Small script that refreshes the kernel feature support status in place.
+#
+
+for F_FILE in Documentation/features/*/*/arch-support.txt; do
+	F=$(grep "^#         Kconfig:" "$F_FILE" | cut -c26-)
+
+	#
+	# Each feature F is identified by a pair (O, K), where 'O' can
+	# be either the empty string (for 'nop') or "not" (the logical
+	# negation operator '!'); other operators are not supported.
+	#
+	O=""
+	K=$F
+	if [[ "$F" == !* ]]; then
+		O="not"
+		K=$(echo $F | sed -e 's/^!//g')
+	fi
+
+	#
+	# F := (O, K) is 'valid' iff there is a Kconfig file (for some
+	# arch) which contains K.
+	#
+	# Notice that this definition entails an 'asymmetry' between
+	# the case 'O = ""' and the case 'O = "not"'. E.g., F may be
+	# _invalid_ if:
+	#
+	# [case 'O = ""']
+	#   1) no arch provides support for F,
+	#   2) K does not exist (e.g., it was renamed/mis-typed);
+	#
+	# [case 'O = "not"']
+	#   3) all archs provide support for F,
+	#   4) as in (2).
+	#
+	# The rationale for adopting this definition (and, thus, for
+	# keeping the asymmetry) is:
+	#
+	#       We want to be able to 'detect' (2) (or (4)).
+	#
+	# (1) and (3) may further warn the developers about the fact
+	# that K can be removed.
+	#
+	F_VALID="false"
+	for ARCH_DIR in arch/*/; do
+		K_FILES=$(find $ARCH_DIR -name "Kconfig*")
+		K_GREP=$(grep "$K" $K_FILES)
+		if [ ! -z "$K_GREP" ]; then
+			F_VALID="true"
+			break
+		fi
+	done
+	if [ "$F_VALID" = "false" ]; then
+		printf "WARNING: '%s' is not a valid Kconfig\n" "$F"
+	fi
+
+	T_FILE="$F_FILE.tmp"
+	grep "^#" $F_FILE > $T_FILE
+	echo "    -----------------------" >> $T_FILE
+	echo "    |         arch |status|" >> $T_FILE
+	echo "    -----------------------" >> $T_FILE
+	for ARCH_DIR in arch/*/; do
+		ARCH=$(echo $ARCH_DIR | sed -e 's/^arch//g' | sed -e 's/\///g')
+		K_FILES=$(find $ARCH_DIR -name "Kconfig*")
+		K_GREP=$(grep "$K" $K_FILES)
+		#
+		# Arch support status values for (O, K) are updated according
+		# to the following rules.
+		#
+		#   - ("", K) is 'supported by a given arch', if there is a
+		#     Kconfig file for that arch which contains K;
+		#
+		#   - ("not", K) is 'supported by a given arch', if there is
+		#     no Kconfig file for that arch which contains K;
+		#
+		#   - otherwise: preserve the previous status value (if any),
+		#                default to 'not yet supported'.
+		#
+		# Notice that, according these rules, invalid features may be
+		# updated/modified.
+		#
+		if [ "$O" = "" ] && [ ! -z "$K_GREP" ]; then
+			printf "    |%12s: |  ok  |\n" "$ARCH" >> $T_FILE
+		elif [ "$O" = "not" ] && [ -z "$K_GREP" ]; then
+			printf "    |%12s: |  ok  |\n" "$ARCH" >> $T_FILE
+		else
+			S=$(grep -v "^#" "$F_FILE" | grep " $ARCH:")
+			if [ ! -z "$S" ]; then
+				echo "$S" >> $T_FILE
+			else
+				printf "    |%12s: | TODO |\n" "$ARCH" \
+					>> $T_FILE
+			fi
+		fi
+	done
+	echo "    -----------------------" >> $T_FILE
+	mv $T_FILE $F_FILE
+done