Re: [PATCH] selftests: rtc: fix flaky date_read_loop test

[Alexandre Belloni]

On 30/04/2026 20:03:13+0800, Wake Liu wrote:
> The test case rtc.date_read_loop in rtctest.c fails intermittently because
> it checks that the RTC time does not advance by more than 1 second per loop
> iteration. However, the loop sleeps for 11ms via nanosleep(), and if the
> test thread is descheduled by the OS scheduler (e.g., under heavy system
> load in a VM), more than 1 second can elapse between consecutive RTC reads.
> This causes the next RTC time read to be 2 or more seconds ahead of the
> previous read, triggering a test assertion failure.
> 
> To make the test more resilient against OS scheduling delays, measure the
> real elapsed time between iterations using a monotonic clock
> (clock_gettime(CLOCK_MONOTONIC)), and compute the actual number of seconds
> elapsed (delta_s) between consecutive RTC reads. Then dynamically adjust
> the assertion to:
> ASSERT_GE(prev_rtc_read + delta_s + 1, rtc_read);
> 
> Signed-off-by: Wake Liu <wakel@xxxxxxxxxx>

Acked-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>



> ---
>  tools/testing/selftests/rtc/rtctest.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/rtc/rtctest.c b/tools/testing/selftests/rtc/rtctest.c
> index 8047d9879039..54eb5c255a45 100644
> --- a/tools/testing/selftests/rtc/rtctest.c
> +++ b/tools/testing/selftests/rtc/rtctest.c
> @@ -116,6 +116,7 @@ TEST_F_TIMEOUT(rtc, date_read_loop, READ_LOOP_DURATION_SEC + 2) {
>  	long iter_count = 0;
>  	struct rtc_time rtc_tm;
>  	time_t start_rtc_read, prev_rtc_read;
> +	struct timespec prev_mono, cur_mono;
>  
>  	if (self->fd == -1 && errno == ENOENT)
>  		SKIP(return, "Skipping test since %s does not exist", rtc_file);
> @@ -126,25 +127,31 @@ TEST_F_TIMEOUT(rtc, date_read_loop, READ_LOOP_DURATION_SEC + 2) {
>  
>  	rc = ioctl(self->fd, RTC_RD_TIME, &rtc_tm);
>  	ASSERT_NE(-1, rc);
> +	clock_gettime(CLOCK_MONOTONIC, &prev_mono);
>  	start_rtc_read = rtc_time_to_timestamp(&rtc_tm);
>  	prev_rtc_read = start_rtc_read;
>  
>  	do  {
>  		time_t rtc_read;
> +		time_t delta_s = 0;
>  
>  		rc = ioctl(self->fd, RTC_RD_TIME, &rtc_tm);
>  		ASSERT_NE(-1, rc);
> +		clock_gettime(CLOCK_MONOTONIC, &cur_mono);
>  
>  		rtc_read = rtc_time_to_timestamp(&rtc_tm);
> +		delta_s = cur_mono.tv_sec - prev_mono.tv_sec;
> +
>  		/* Time should not go backwards */
>  		ASSERT_LE(prev_rtc_read, rtc_read);
> -		/* Time should not increase more then 1s at a time */
> -		ASSERT_GE(prev_rtc_read + 1, rtc_read);
> +		/* Time should not increase more then elapsed time + 1s */
> +		ASSERT_GE(prev_rtc_read + delta_s + 1, rtc_read);
>  
>  		/* Sleep 11ms to avoid killing / overheating the RTC */
>  		nanosleep_with_retries(READ_LOOP_SLEEP_MS * 1000000);
>  
>  		prev_rtc_read = rtc_read;
> +		prev_mono = cur_mono;
>  		iter_count++;
>  	} while (prev_rtc_read <= start_rtc_read + READ_LOOP_DURATION_SEC);
>  
> -- 
> 2.54.0.545.g6539524ca2-goog
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com