[LTP] [PATCH v2] madvise06: shrink to 3 MADV_WILLNEED pages to stabilize the test

* [LTP] [PATCH v2] madvise06: shrink to 3 MADV_WILLNEED pages to stabilize the test
@ 2022-06-21  3:47 Li Wang
  2022-06-21  8:27 ` Richard Palethorpe
  0 siblings, 1 reply; 3+ messages in thread
From: Li Wang @ 2022-06-21  3:47 UTC (permalink / raw)
  To: ltp; +Cc: Paul Bunyan, Rafael Aquini, Richard Palethorpe

Paul Bunyan reports that the madvise06 test fails intermittently with many
LTS kernels, after checking with mm developer we prefer to think this is
more like a test issue (but not kernel bug):

   madvise06.c:231: TFAIL: 4 pages were faulted out of 2 max

So this improvement is target to reduce the false positive happens from
three points:

  1. Adding the while-loop to give more chances for madvise_willneed()
     reads memory asynchronously
  2. Raise value of `loop` to let test waiting for more times if swapchache
     haven't reached the expected
  3. Shrink to only 3 pages for verifying MADV_WILLNEED that to make the
     system easily takes effect on it

From Rafael Aquini:

  The problem here is that MADV_WILLNEED is an asynchronous non-blocking
  hint, which will tell the kernel to start doing read-ahead work for the
  hinted memory chunk, but will not wait up for the read-ahead to finish.
  So, it is possible that when the dirty_pages() call start re-dirtying
  the pages in that target area, is racing against a scheduled swap-in
  read-ahead that hasn't yet finished. Expecting faulting only 2 pages
  out of 102400 also seems too strict for a PASS threshold.

Note:
  As Rafael suggested, another possible approach to tackle this failure
  is to tally up, and loosen the threshold to more than 2 major faults
  after a call to madvise() with MADV_WILLNEED.
  But from my test, seems the faulted-out page shows a significant
  variance in different platforms, so I didn't take this way.

Btw, this patch get passed on my two easy reproducible systems more than 1000 times

Reported-by: Paul Bunyan <pbunyan@redhat.com>
Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Richard Palethorpe <rpalethorpe@suse.com>
---
 testcases/kernel/syscalls/madvise/madvise06.c | 21 +++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/testcases/kernel/syscalls/madvise/madvise06.c b/testcases/kernel/syscalls/madvise/madvise06.c
index 6d218801c..27aff18f1 100644
--- a/testcases/kernel/syscalls/madvise/madvise06.c
+++ b/testcases/kernel/syscalls/madvise/madvise06.c
@@ -164,7 +164,7 @@ static int get_page_fault_num(void)
 
 static void test_advice_willneed(void)
 {
-	int loops = 50, res;
+	int loops = 100, res;
 	char *target;
 	long swapcached_start, swapcached;
 	int page_fault_num_1, page_fault_num_2;
@@ -202,23 +202,32 @@ static void test_advice_willneed(void)
 		"%s than %ld Kb were moved to the swap cache",
 		res ? "more" : "less", PASS_THRESHOLD_KB);
 
-
-	TEST(madvise(target, PASS_THRESHOLD, MADV_WILLNEED));
+	loops = 100;
+	SAFE_FILE_LINES_SCANF("/proc/meminfo", "SwapCached: %ld", &swapcached_start);
+	TEST(madvise(target, pg_sz * 3, MADV_WILLNEED));
 	if (TST_RET == -1)
 		tst_brk(TBROK | TTERRNO, "madvise failed");
+	do {
+		loops--;
+		usleep(100000);
+		if (stat_refresh_sup)
+			SAFE_FILE_PRINTF("/proc/sys/vm/stat_refresh", "1");
+		SAFE_FILE_LINES_SCANF("/proc/meminfo", "SwapCached: %ld",
+				&swapcached);
+	} while (swapcached < swapcached_start + pg_sz*3/1024 && loops > 0);
 
 	page_fault_num_1 = get_page_fault_num();
 	tst_res(TINFO, "PageFault(madvice / no mem access): %d",
 			page_fault_num_1);
-	dirty_pages(target, PASS_THRESHOLD);
+	dirty_pages(target, pg_sz * 3);
 	page_fault_num_2 = get_page_fault_num();
 	tst_res(TINFO, "PageFault(madvice / mem access): %d",
 			page_fault_num_2);
 	meminfo_diag("After page access");
 
 	res = page_fault_num_2 - page_fault_num_1;
-	tst_res(res < 3 ? TPASS : TFAIL,
-		"%d pages were faulted out of 2 max", res);
+	tst_res(res == 0 ? TPASS : TFAIL,
+		"%d pages were faulted out of 3 max", res);
 
 	SAFE_MUNMAP(target, CHUNK_SZ);
 }
-- 
2.35.3


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