[1/2] libnewipc: Add get_ipc_timestamp()

That returns timestamps that should return values comparable to the
stime/rtime/ctime.

From Thomas Gleixner:

> Due to the internal implementation of ktime_get_real_seconds(), which is
> a 2038 safe replacement for the former get_seconds() function, this
> accumulation issue can be observed. (time(2) via syscall and newer
> versions of VDSO use the same mechanism).
>
>      clock_gettime(CLOCK_REALTIME, &ts);
>      sec = time();
>      assert(sec >= ts.tv_sec);
>
> That assert can trigger for two reasons:
>
>  1) Clock was set between the clock_gettime() and time().
>
>  2) The clock has advanced far enough that:
>
>     timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC
> The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE
>
>      clock_gettime(CLOCK_XXX, &ts);
>      clock_gettime(CLOCK_XXX_COARSE, &tc);
>      assert(tc.tv_sec >= ts.tv_sec);
>
> The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec
> pair without reading the clock. Same as #2 above just extended to clock
> MONOTONIC.

Which means the timestamps in the structure we get from msg* calls can
be 1 second smaller that timestamps returned from realtime timers.

However it also means that the _COARSE timers should be the same as the
SysV timestamps and could be used for these tests instead. Also these
timers should be available since 2.6.32 which is old enough to assume
that they are present.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
---
 include/libnewipc.h           |  3 +++
 libs/libltpnewipc/libnewipc.c | 13 +++++++++++++
 2 files changed, 16 insertions(+)

Hi Cyril,

On Thu, Dec 3, 2020 at 11:27 PM Cyril Hrubis <chrubis@suse.cz> wrote:

> That returns timestamps that should return values comparable to the
> stime/rtime/ctime.
>
> From Thomas Gleixner:
>
> > Due to the internal implementation of ktime_get_real_seconds(), which is
> > a 2038 safe replacement for the former get_seconds() function, this
> > accumulation issue can be observed. (time(2) via syscall and newer
> > versions of VDSO use the same mechanism).
> >
> >      clock_gettime(CLOCK_REALTIME, &ts);
> >      sec = time();
> >      assert(sec >= ts.tv_sec);
> >
> > That assert can trigger for two reasons:
> >
> >  1) Clock was set between the clock_gettime() and time().
> >
> >  2) The clock has advanced far enough that:
> >
> >     timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC
> > The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE
> >
> >      clock_gettime(CLOCK_XXX, &ts);
> >      clock_gettime(CLOCK_XXX_COARSE, &tc);
> >      assert(tc.tv_sec >= ts.tv_sec);
> >
> > The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec
> > pair without reading the clock. Same as #2 above just extended to clock
> > MONOTONIC.
>
> Which means the timestamps in the structure we get from msg* calls can
> be 1 second smaller that timestamps returned from realtime timers.
>
> However it also means that the _COARSE timers should be the same as the
> SysV timestamps and could be used for these tests instead. Also these
> timers should be available since 2.6.32 which is old enough to assume
> that they are present.
>

Sounds reasonable. From Thomas's analysis, it seems the *_COARSE
can work for us, so I'm OK to go with this way to fix.

For series:
Acked-by: Li Wang <liwang@redhat.com>