getrusage03: Forcing context switches to update resource usage

Our CI sporadically complains that this test grandchild's MAXRSS did not reach
the expected 300MB size.

    12 getrusage03.c:86: TFAIL: child.children = 258048, expected 307200

As the ru_maxrss value is generally updated at certain intervals or under
specific conditions, such as page faults or context switches. There may be
delay between the completion of memset() and the update of ru_maxrss.

To address this issue, we create a function to force context switches by
calling sched_yield() multiple times. This approach helps to ensure that
the system has the opportunity to update the ru_maxrss value more promptly.

Reproted-by: Scott Weaver <[email protected]>
Signed-off-by: Li Wang <[email protected]>
Reviewed-by: Cyril Hrubis <[email protected]>

testcases/kernel/syscalls/getrusage/getrusage03.h

@@ -6,10 +6,19 @@
 #ifndef LTP_GETRUSAGE03_H
 #define LTP_GETRUSAGE03_H
 
+#include <sched.h>
 #include "tst_test.h"
 
 #define DELTA_MAX 20480
 
+static void force_context_switches(int iterations)
+{
+	tst_res(TINFO, "Forcing context switch %d times", iterations);
+
+	for (int i = 0; i < iterations; i++)
+		sched_yield();
+}
+
 static void consume_mb(int consume_nr)
 {
 	void *ptr;
@@ -22,6 +31,8 @@ static void consume_mb(int consume_nr)
 	ptr = SAFE_MALLOC(size);
 	memset(ptr, 0, size);
 
+	force_context_switches(10);
+
 	SAFE_FILE_LINES_SCANF("/proc/self/status", "VmSwap: %lu", &vmswap_size);
 	if (vmswap_size > 0)
 		tst_brk(TBROK, "VmSwap is not zero");