Re: How should we do a 64-bit jiffies?

[Brian Gerst]

Keith Owens wrote:
> 
> On Mon, 22 Oct 2001 08:12:24 -0700,
> george anzinger <[e-mail]> wrote:
> >I am working on POSIX timers where there is defined a CLOCK_MONOTONIC.
> >The most reasonable implementation of this clock is that it is "uptime"
> >or jiffies.  The problem is that it is most definitely not MONOTONIC
> >when it rolls back to 0 :(  Thus the need for 64-bits.
> 
> If you want to leave existing kernel code alone so it still uses 32 bit
> jiffies, just maintain a separate high order 32 bit field which is only
> used by the code that really needs it.  On 32 bit machines, the jiffie
> code does
> 
>   old_jiffies = jiffies++;
>   if (jiffies < old_jiffies)
>         ++high_jiffies;
> 
> You will need a spin lock around that on 32 bit systems, but that is
> true for anything that tries to do 64 bit counter updates on a 32 bit
> system.  None of your suggestions will work on ix86, it does not
> support atomic updates on 64 bit fields in hardware.

cmpxchg8b does, but it's a bit indirect.

-- 

						Brian Gerst