[LTP] [PATCH] syscalls/clock_gettime: Add test to check bug during successive readings

[Viresh Kumar]

On 09-06-20, 09:43, Arnd Bergmann wrote:
> On Tue, Jun 9, 2020 at 9:05 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >
> > On 08-06-20, 16:51, Arnd Bergmann wrote:
> > > You already have a special case for gettimeofday(), so just round down
> > > the 'start' value to the previous microsecond when you are in that
> > > case.
> >
> > Since this will also affect the next reading as well (as we will loose
> > values < 1 us), I tried to do it a bit differently.
> >
> > @@ -78,6 +78,13 @@ static void run(unsigned int i)
> >                         tv->gettime(clks[i], tst_ts_get(&ts));
> >                         end = tst_ts_to_ns(ts);
> >
> > +                       /*
> > +                        * gettimeofday() doesn't value less than 1 us, copy
> > +                        * that from start.
> > +                        */
> > +                       if (tv->gettime == my_gettimeofday && end < start)
> > +                               end += start % 1000;
> 
> This looks like it should work correctly, but it feels a bit more fragile than
> 
>                        if (tv->gettime == my_gettimeofday)
>                                start -= start % 1000;
> 
> which would bring the start and end values to the same resolution
> rather than making up values that were never read.

This sounds better, will pick it up.

> Your approach however has the advantage of adding less overhead
> as the % operator on a 64-bit integer is going to turn into a very
> expensive calculation on most 32-bit processors that itself can
> skew the timing.

I am not sure of what you meant here, both the approaches are using
the % operation.

-- 
viresh