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

[Keith Owens]

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.