[PATCH 2/3] selftests/sched_ext: Fix dsq_reenq test reliability

[zhidao su]

The dsq_reenq test was using NUM_WORKERS=4 on a 4-CPU system, so
each worker ran on its own CPU. Workers sleep 500us and yield,
getting dispatched from USER_DSQ almost immediately. By the time
the BPF timer's deferred scx_bpf_dsq_reenq() fires, USER_DSQ is
empty and no reenqueue event is observed (nr_reenq_kfunc stays 0).

Fix by increasing NUM_WORKERS to 16 (>4 CPUs). With more runnable
tasks than CPUs, USER_DSQ always has a backlog when the timer fires,
so scx_bpf_dsq_reenq() reliably finds tasks to reenqueue.

Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
---
 tools/testing/selftests/sched_ext/dsq_reenq.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/sched_ext/dsq_reenq.c b/tools/testing/selftests/sched_ext/dsq_reenq.c
index b0d99f9c9a9a..fe18c6af3877 100644
--- a/tools/testing/selftests/sched_ext/dsq_reenq.c
+++ b/tools/testing/selftests/sched_ext/dsq_reenq.c
@@ -15,7 +15,12 @@
 #include "dsq_reenq.bpf.skel.h"
 #include "scx_test.h"
 
-#define NUM_WORKERS	4
+/*
+ * Use more workers than CPUs so USER_DSQ is always backlogged.
+ * With tasks queued faster than dispatch can consume them, the
+ * periodic timer will always find tasks to reenqueue.
+ */
+#define NUM_WORKERS	16
 #define TEST_DURATION_SEC 3
 
 static volatile bool stop_workers;
-- 
2.43.0