[Devel] [PATCH vz10 1/3] ms/selftests/posix_timers: Use CLOCK_THREAD_CPUTIME_ID for ITIMER_PROF measurements

[Eva Kurchatova]

From: John Stultz <jstultz at google.com>

It was reported that the posix_timers test was at times seeing failures
with ITIMER_PROF timers, specifically in cases where the RCU_SOFTIRQ was
taking up significant amounts of time.

Analysis showed that as the time in softirq isn't included in the task
stime + utime accounting used to trigger the SIGPROF so delays from softirq
work could cause it to appear that the signal was incorrectly delayed.

Contributing to this is that the test uses gettimeofday() to measure
itimers, which also means any scheduling delay can also cause failures (as
the task may not be running the entire time).

To fix this, convert all the itimer measurements to use clock_gettime(),
tweaking the logic to use nsecs instead of usecs. Then for ITIMER_PROF
timers, utilize the CLOCK_THREAD_CPUTIME_ID clockid so that it is similarly
measuring the time the task was running.

Signed-off-by: John Stultz <jstultz at google.com>
Signed-off-by: Thomas Gleixner <tglx at kernel.org>
Link: https://patch.msgid.link/20260428173957.1394265-1-jstultz@google.com
The ITIMER_PROF case of it fails on the CI machines, which are two
cpu guests under load:

  # Diff too high: 2561992..not ok 3 ITIMER_PROF

2.56 seconds of wall time for the 2 seconds of cpu time the timer
counts, so the task had about four fifths of a cpu.

(cherry picked from commit b00385b8d081ce74f36ea178e04e1b106505fb36)

https://virtuozzo.atlassian.net/browse/VSTOR-143363
Feature: fix selftests
Signed-off-by: Eva Kurchatova <eva.kurchatova at virtuozzo.com>
---
 tools/testing/selftests/timers/posix_timers.c | 55 ++++++++++---------
 1 file changed, 28 insertions(+), 27 deletions(-)

diff --git a/tools/testing/selftests/timers/posix_timers.c b/tools/testing/selftests/timers/posix_timers.c
index f0eceb0faf34..077a98fb1b1d 100644
--- a/tools/testing/selftests/timers/posix_timers.c
+++ b/tools/testing/selftests/timers/posix_timers.c
@@ -77,19 +77,25 @@ static void sig_handler(int nr)
 	done = 1;
 }
 
+static inline int64_t calcdiff_ns(struct timespec t1, struct timespec t2)
+{
+	int64_t diff;
+
+	diff = NSEC_PER_SEC * (int64_t)((int) t1.tv_sec - (int) t2.tv_sec);
+	diff += ((int) t1.tv_nsec - (int) t2.tv_nsec);
+	return diff;
+}
+
 /*
  * Check the expected timer expiration matches the GTOD elapsed delta since
  * we armed the timer. Keep a 0.5 sec error margin due to various jitter.
  */
-static int check_diff(struct timeval start, struct timeval end)
+static int check_diff(struct timespec start, struct timespec end)
 {
-	long long diff;
-
-	diff = end.tv_usec - start.tv_usec;
-	diff += (end.tv_sec - start.tv_sec) * USEC_PER_SEC;
+	long long diff = calcdiff_ns(end, start);
 
-	if (llabs(diff - DELAY * USEC_PER_SEC) > USEC_PER_SEC / 2) {
-		printf("Diff too high: %lld..", diff);
+	if (llabs(diff - DELAY * NSEC_PER_SEC) > NSEC_PER_SEC / 2) {
+		printf("Diff too high: %lld ns..", diff);
 		return -1;
 	}
 
@@ -98,22 +104,25 @@ static int check_diff(struct timeval start, struct timeval end)
 
 static void check_itimer(int which, const char *name)
 {
-	struct timeval start, end;
+	struct timespec start, end;
 	struct itimerval val = {
 		.it_value.tv_sec = DELAY,
 	};
+	int clock_id = CLOCK_REALTIME;
 
 	done = 0;
 
 	if (which == ITIMER_VIRTUAL)
 		signal(SIGVTALRM, sig_handler);
-	else if (which == ITIMER_PROF)
+	else if (which == ITIMER_PROF) {
+		clock_id = CLOCK_THREAD_CPUTIME_ID;
 		signal(SIGPROF, sig_handler);
+	}
 	else if (which == ITIMER_REAL)
 		signal(SIGALRM, sig_handler);
 
-	if (gettimeofday(&start, NULL) < 0)
-		fatal_error(name, "gettimeofday()");
+	if (clock_gettime(clock_id, &start))
+		fatal_error(name, "clock_gettime()");
 
 	if (setitimer(which, &val, NULL) < 0)
 		fatal_error(name, "setitimer()");
@@ -125,18 +134,19 @@ static void check_itimer(int which, const char *name)
 	else if (which == ITIMER_REAL)
 		idle_loop();
 
-	if (gettimeofday(&end, NULL) < 0)
-		fatal_error(name, "gettimeofday()");
+	if (clock_gettime(clock_id, &end))
+		fatal_error(name, "clock_gettime()");
 
 	ksft_test_result(check_diff(start, end) == 0, "%s\n", name);
 }
 
 static void check_timer_create(int which, const char *name)
 {
-	struct timeval start, end;
+	struct timespec start, end;
 	struct itimerspec val = {
 		.it_value.tv_sec = DELAY,
 	};
+	int clock_id = CLOCK_REALTIME;
 	timer_t id;
 
 	done = 0;
@@ -147,16 +157,16 @@ static void check_timer_create(int which, const char *name)
 	if (signal(SIGALRM, sig_handler) == SIG_ERR)
 		fatal_error(name, "signal()");
 
-	if (gettimeofday(&start, NULL) < 0)
-		fatal_error(name, "gettimeofday()");
+	if (clock_gettime(clock_id, &start))
+		fatal_error(name, "clock_gettime()");
 
 	if (timer_settime(id, 0, &val, NULL) < 0)
 		fatal_error(name, "timer_settime()");
 
 	user_loop();
 
-	if (gettimeofday(&end, NULL) < 0)
-		fatal_error(name, "gettimeofday()");
+	if (clock_gettime(clock_id, &end))
+		fatal_error(name, "clock_gettime()");
 
 	ksft_test_result(check_diff(start, end) == 0,
 			 "timer_create() per %s\n", name);
@@ -444,15 +454,6 @@ static void check_delete(void)
 	ksft_test_result(!tsig.signals, "check_delete\n");
 }
 
-static inline int64_t calcdiff_ns(struct timespec t1, struct timespec t2)
-{
-	int64_t diff;
-
-	diff = NSEC_PER_SEC * (int64_t)((int) t1.tv_sec - (int) t2.tv_sec);
-	diff += ((int) t1.tv_nsec - (int) t2.tv_nsec);
-	return diff;
-}
-
 static void check_sigev_none(int which, const char *name)
 {
 	struct timespec start, now;
-- 
2.55.0