Re: [Xenomai-core] [PATCH] Use tsc for implementation of clock_gettime.

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Gilles Chanteperdrix wrote:
>  > Jan Kiszka wrote:
>  >  > Gilles Chanteperdrix wrote:
>  >  > > Jan Kiszka wrote:
>  >  > >  > Gilles Chanteperdrix wrote:
>  >  > >  > > Jan Kiszka wrote:
>  >  > >  > >  > Gilles Chanteperdrix wrote:
>  >  > >  > >  > > Hi,
>  >  > >  > >  > > 
>  >  > >  > >  > > here comes, for review, a patch which reduces the overhead of
>  >  > >  > >  > > clock_gettime by directly reading the tsc in user-space for
>  >  > >  > >  > > architectures that support it.
>  >  > >  > >  > 
>  >  > >  > >  > Highly welcome. But I have one concern: How and when do you propagate
>  >  > >  > >  > wallclock_offset changes to user space?
>  >  > >  > > 
>  >  > >  > > Since clock_settime is not implemented, never, but if clock_settime was
>  >  > >  > > implemented, clock_settime would re-issue the __xn_sys_info syscall.
>  >  > >  > 
>  >  > >  > This excludes automatic clock adjustment, something I'm convinced we
>  >  > >  > will have to provide in the future.
>  >  > > 
>  >  > > When we provide automatic clock adjustment, we will have to devise
>  >  > > something more subtle than just an offset, so we will have to redesign
>  >  > 
>  >  > I think offset + scaling factor.
>  >  > 
>  >  > > posix clocks support anyway. Maybe clock_gettime(CLOCK_REALTIME) will
>  >  > > then always be a syscall. After all, rt_timer_read is a syscall. If you
>  >  > > want the fast clock, use CLOCK_MONOTONIC or rt_timer_tsc.
>  >  > 
>  >  > Actually, the issue of the intermediate approach starts earlier: as soon
>  >  > as you have clock_settime. Then you need to sync the offset across
>  >  > applications in different processes.
>  >  > 
>  >  > Having clock_monotonic (and maybe also rt_timer_tsc2ns) as a fast
>  >  > variant already now is not bad. But beyond that, before introducing a
>  >  > new interface between kernel and user space, I would like to consider
>  >  > the effort to get it future-proof immediately. That doesn't mean that we
>  >  > already have to implement clock adjustment, but we may already prepare
>  >  > the prerequisites.
>  > 
>  > Ok. Here is a new version that does not break the ABI, and where
>  > clock_gettime(CLOCK_REALTIME) remains a syscall.
>  > 
>  > Note that it also adds user-space conversions between tsc and ns to the
>  > native skin.
> 
> This one is actually tested.
> 

Looks good to me, builds and runs fine here.

Jan


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