[PATCH v2] selftests: cpufreq: stop CPU hotplug loop after cpufreq update test

[xiong yining]

Original test uses a fixed 5000 iteration loop to trigger CPU hotplug
for verifying cpufreq synchronization between CPU hotplug and sysfs
writes.

Two issues exist:
1. Background hotplug task keeps running after test completes, causing
redundant CPU on/offline operations.
2. When cpufreq frequency setup is slow, the hotplug loop may exit
before frequency updates finish, which breaks test purpose.

Introduce a file-based stop flag to control the infinite hotplug loop.
After all cpufreq updates complete, create the stop flag file and
wait for the hotplug task to exit.

This makes hotplug run only during cpufreq sysfs write test,
align with test objective.

Signed-off-by: xiong yining <xiongyining1480@xxxxxxxxxxxxxx>
---
Changes in v2:
- Stop the hotplug child process by signal notification instead of
a stop file, avoiding the predictable temporary path.
- Add an exit trap so the hotplug child is also cleaned up when the
main shell terminates abnormally.
- Link to v1:
https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2662696.html

This addresses the security and robustness concerns raised by Sashiko.
---
 .../selftests/cpufreq/special-tests.sh        | 30 +++++++++++++++----
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/cpufreq/special-tests.sh b/tools/testing/selftests/cpufreq/special-tests.sh
index e87ed7c8e5e5..b3ebead1c52a 100755
--- a/tools/testing/selftests/cpufreq/special-tests.sh
+++ b/tools/testing/selftests/cpufreq/special-tests.sh
@@ -16,6 +16,8 @@ source cpu.sh
 source cpufreq.sh
 source governor.sh
 
+HOTPLUG_PID=""
+
 # Test 1
 # $1: policy
 __simple_lockdep()
@@ -92,20 +94,34 @@ governor_race()
 	restore_all_governors
 }
 
+hotplug_cleanup()
+{
+	if [[ -n "$HOTPLUG_PID" ]]; then
+		kill -s SIGTERM "$HOTPLUG_PID"
+		wait "$HOTPLUG_PID"
+
+		HOTPLUG_PID=""
+	fi
+}
+
 # Test 4
 # $1: cpu
 hotplug_with_updates_cpu()
 {
 	local filepath="$CPUROOT/$1/cpufreq"
 
+	(
+		local quit=0
+		trap 'quit=1' SIGTERM
+		while [ "$quit" -eq 0 ]; do
+			reboot_cpu "$1"
+		done
+	) &
+	HOTPLUG_PID=$!
+
 	# switch to ondemand
 	__switch_governor_for_cpu $1 "ondemand"
 
-	for i in `seq 1 5000`
-	do
-		reboot_cpu $1
-	done &
-
 	local freqs=$(cat $filepath/scaling_available_frequencies)
 	local oldfreq=$(cat $filepath/scaling_min_freq)
 
@@ -117,14 +133,16 @@ hotplug_with_updates_cpu()
 		done
 	done
 
+	hotplug_cleanup
+
 	# restore old freq
 	echo $oldfreq > $filepath/scaling_min_freq
 }
 
 hotplug_with_updates()
 {
+	trap 'hotplug_cleanup' EXIT
 	save_all_governors
 	for_each_non_boot_cpu hotplug_with_updates_cpu
-	wait
 	restore_all_governors
 }
-- 
2.34.1