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

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Message-ID: <6811994.pqkAjrM8xH@wuerfel>
Date:	Wed, 14 May 2014 16:01:05 +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 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.

> And for a 32bit kernel you have two
> 
> SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
>                 struct timespec __user *, utime, u32 __user *, uaddr2,
>                 u32, val3)
> 
> SYSCALL_DEFINE6(futex64, u32 __user *, uaddr, int, op, u32, val,
>                 struct timespec64 __user *, utime, u32 __user *, uaddr2,
>                 u32, val3)

And here, we could think about renaming the kernel internal symbols
so that we use the same two definitions as well, one for compat_timespec
and one for timespec64. Those are details we don't need to debate now.

> Fine with me, but we really need to discuss the timespec64 with user
> space folks.

Definitely.

> 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.
FWIW, 64-bit ns gives us 584 years worth of nanoseconds, which
means none of us or the people we know will be around before this
becomes a problem ;-)

For the user interface, we can decide which representation to use
for each syscall individually depending on the needs. We should just
not have to many different variants. I was going for timespec64
just because that would allow us to keep the 64-bit kernel ABI
unchanged.

> I often enough cursed timespec for clock_nanosleep on an absolute
> timeline. I need to go through all that normalizing stuff instead of
> just doing next_event += 500000;

And all user space needs to do it too.

	Arnd
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