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

[Wake Liu]

Hi Thomas,

Thanks for the review. I've addressed all the style issues and sent
out PATCH v2.

On Mon, Jul 20, 2026 at 10:14 PM Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
> 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.
>


-- 
Best Regards,
Wake Liu