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

[george anzinger <george@mvista.com>]

Keith Owens wrote:
> 
> On Mon, 22 Oct 2001 08:12:24 -0700,
> george anzinger <george@mvista.com> 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.

As it turns out I already have a spinlock on the update jiffies code. 
The reason one would want to use a 64-bit integer is that the compiler
does a MUCH better job of the ++, i.e. it just does an add carry.  No
if, no jmp.  I suppose I need to lock the read also, but it is not done
often and will hardly ever block.

I am beginning to think that defining a u64 and casting, i.e.:

#define jiffies (unsigned long volitial)jiffies_u64

is the way to go.

George