Re: [LTP] [PATCH] clock_gettime04: set threshold based on the clock tick rate

[Waiman Long <longman@redhat.com>]

On 3/25/22 00:00, Li Wang wrote:
> This is to get rid of the intermittent failures in clock_gettime04,
> which are likely caused by different clock tick rates on platforms.
> Here set the threshold no less than each clock tick in millisecond:
>
> 	delta = 1000(ms)/ticks_number_per_sec + 5;
>
> Error log:
>    clock_gettime04.c:163: TFAIL: CLOCK_REALTIME_COARSE(syscall with old kernel spec):
>          Difference between successive readings greater than 5 ms (1): 10
>    clock_gettime04.c:163: TFAIL: CLOCK_MONOTONIC_COARSE(vDSO with old kernel spec):
> 	Difference between successive readings greater than 5 ms (2): 10
>
>  From Waiman Long:
>    That failure happens for CLOCK_REALTIME_COARSE which is a faster but less
>    precise version of CLOCK_REALTIME. The time resolution is actually a clock
>    tick. Since arm64 has a HZ rate of 100. That means each tick is 10ms. So a
>    CLOCK_REALTIME_COARSE threshold of 5ms is probably not enough. I would say
>    in the case of CLOCK_REALTIME_COARSE, we have to increase the threshold based
>    on the clock tick rate of the system. This is more a test failure than is
>    an inherent problem in the kernel.
>
> Fixes #898
>
> Reported-by: Eirik Fuller <efuller@redhat.com>
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: Waiman Long <llong@redhat.com>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>   testcases/kernel/syscalls/clock_gettime/clock_gettime04.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/testcases/kernel/syscalls/clock_gettime/clock_gettime04.c b/testcases/kernel/syscalls/clock_gettime/clock_gettime04.c
> index a8d2c5b38..cccbc9383 100644
> --- a/testcases/kernel/syscalls/clock_gettime/clock_gettime04.c
> +++ b/testcases/kernel/syscalls/clock_gettime/clock_gettime04.c
> @@ -35,7 +35,7 @@ clockid_t clks[] = {
>   };
>   
>   static gettime_t ptr_vdso_gettime, ptr_vdso_gettime64;
> -static long long delta = 5;
> +static long long delta;
>   
>   static inline int do_vdso_gettime(gettime_t vdso, clockid_t clk_id, void *ts)
>   {
> @@ -92,6 +92,7 @@ static struct time64_variants variants[] = {
>   
>   static void setup(void)
>   {
> +	delta = 1000/sysconf(_SC_CLK_TCK) + 5;
>   	if (tst_is_virt(VIRT_ANY)) {
>   		tst_res(TINFO, "Running in a virtual machine, multiply the delta by 10.");
>   		delta *= 10;

Actually, only CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE use 
clock tick for its time measurement. The rests use a higher resolution 
internal timer. Perhaps, we could have a separate coarse_delta for these 
two cases and use delta for the rest.

Cheers,
Longman


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