linux-kernel - Re: [PATCH 00/25] Change time_t and clock

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Date:	Wed, 14 May 2014 17:48:33 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

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
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