Re: [PATCH] selftests: timers: Use CPU time for CPU timers in posix_timers test

[Thomas Gleixner <tglx@kernel.org>]

On Wed, Jul 08 2026 at 12:15, Wake Liu wrote:
> The posix_timers test compares the elapsed real time (GTOD) with the
> expected timer delay to verify if the timer expired. This works fine for
> real-time timers (ITIMER_REAL), but is flaky for CPU-time timers
> (ITIMER_PROF, ITIMER_VIRTUAL, and CLOCK_*_CPUTIME_ID) on loaded systems
> (like shared test runners) because the test process might be scheduled
> out, resulting in elapsed real time being larger than the consumed CPU
> time.
>
> Fix this by using the appropriate measurement method for each timer:
> - Use CPU process/thread time (via clock_gettime or getrusage) for CPU
>   timers.
> - Keep using monotonic clock (real time) for real-time timers.
>
> This makes the test robust against scheduling delays.

Lacks Signed-off-by ....

> +#include <sys/resource.h>
> +
> +static long long get_clock_time_us(clockid_t clock_id)
> +{
> +	struct timespec ts;

Newline between declaration and code.

> +	if (clock_gettime(clock_id, &ts) < 0)
> +		return -1;
> +	return (long long)ts.tv_sec * USECS_PER_SEC + ts.tv_nsec / 1000;
> +}
> +
> +static long long get_itimer_time_us(int which)
> +{
> +	struct rusage usage;
> +
> +	switch (which) {
> +	case ITIMER_REAL:
> +		return get_clock_time_us(CLOCK_MONOTONIC);
> +	case ITIMER_PROF:
> +		return get_clock_time_us(CLOCK_PROCESS_CPUTIME_ID);
> +	case ITIMER_VIRTUAL:
> +		if (getrusage(RUSAGE_SELF, &usage) < 0)
> +			return -1;
> +		return (long long)usage.ru_utime.tv_sec * USECS_PER_SEC + usage.ru_utime.tv_usec;
> +	default:
> +		return -1;
> +	}
> +}
> +
> +static int check_diff_us(long long start, long long end)
>  {
>  	long long diff;
>  
> -	diff = end.tv_usec - start.tv_usec;
> -	diff += (end.tv_sec - start.tv_sec) * USECS_PER_SEC;
> +	diff = end - start;

Move that to the declaration line.

>  	if (llabs(diff - DELAY * USECS_PER_SEC) > USECS_PER_SEC / 2) {
>  		printf("Diff too high: %lld..", diff);
> @@ -100,7 +123,7 @@ static int check_diff(struct timeval start, struct timeval end)
>  
> -	if (gettimeofday(&end, NULL) < 0)
> -		fatal_error(name, "gettimeofday()");
> +	end = get_clock_time_us(which);
> +	if (end < 0)
> +		fatal_error(name, "get_clock_time_us()");
>  
> -	ksft_test_result(check_diff(start, end) == 0,
> +	ksft_test_result(check_diff_us(start, end) == 0,
>  			 "timer_create() per %s\n", name);

Please get rid of the line break.