Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

Arnd Bergmann
2014-05-14T15:49:01Z

On Wednesday 14 May 2014 16:46:29 Thomas Gleixner wrote:
> On Wed, 14 May 2014, Arnd Bergmann wrote:
>
> > On Wednesday 14 May 2014 14:21:48 Thomas Gleixner wrote:
> > >
> > > So in the 32-on-64 case we'll have two compat variants:
> > >
> > > SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
> > > struct timespec __user *, utime, u32 __user *, uaddr2,
> > > u32, val3)
> > >
> > > COMPAT_SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
> > > struct compat_timespec __user *, utime, u32 __user *, uaddr2,
> > > u32, val3)
> > >
> > > COMPAT_SYSCALL_DEFINE6(futex64, u32 __user *, uaddr, int, op, u32, val,
> > > struct timespec64 __user *, utime, u32 __user *, uaddr2,
> > > u32, val3)
> > >
> > > The native 64bit futex64 syscall is mapped to futex.
> >
> > I was actually hoping that we could map the compat futex64 to futex
> > as well here, since 64-bit timespec and compat timespec64 would be
> > the same structure.
>
> Right, that might work with this one, but not for anything which has a
> pointer to a timespec and some other argument based on long.

We'd have to look at each one individually of course, but out of this
list, I think it will work for most:

- getitimer/setitimer
- timer_settime/timer_gettime
- gettimeofday/settimeofday
- adjtimex
- clock_gettime/clock_settime/clock_adjtime
- time/stime
- sysv ipc (msg, sem, shm)
- fstatat (or the statx replacement once we get to that)
- futimesat

but not these ioctls:

- socket time stamps
- audio time stamps
- v4l time stamps
- input event time stamps

> > > I'm curious, whether quite some code, like high frequency timestamps
> > > wouldn't be better of with a strict 64 bit nanosecond granular time
> > > represenation.
> >
> > At least in the kernel, I think ktime_t is already the right type
> > to use on both 64-bit and 32-bit architectures as it can be slow to
> > extract the seconds portion of 64-bit nanoseconds on a 32-bit machine.
>
> On some of them yes. On i386 the u64 nsec ktime_t variant is way more
> efficient.

Ah, interesting. Do we have actual data for other architectures? If
it's much better on x86 and never much worse anywhere else, that
alone may be a reason to migrate the timestamp interfaces.

Arnd