Re: widening ticks

On Wed, Jan 08, 2025 at 04:31:16PM -0500, Mark Johnston wrote:
> The global "ticks" variable counts hardclock ticks, it's widely used in
> the kernel for low-precision timekeeping.  The linuxkpi provides a very
> similar variable, "jiffies", but there's an incompatibility: the former
> is a signed int and the latter is an unsigned long.  It's not
> particularly easy to paper over this difference, which has been
> responsible for some nasty bugs, and modifying drivers to store the
> jiffies value in a signed int is error-prone and a maintenance burden
> that the linuxkpi is supposed to avoid.
> 
> It would be nice to provide a compatible implementation of jiffies.  I
> can see a few approaches:
> - Define a 64-bit ticks variable, say ticks64, and make hardclock()
>   update both ticks and ticks64.  Then #define jiffies ticks64 on 64-bit
>   platforms.  This is the simplest to implement, but it adds extra work
>   to hardclock() and is somewhat ugly.
> - Make ticks an int64_t or a long and convert our native code
>   accordingly.  This is cleaner but requires a lot of auditing to avoid
>   introducing bugs, though perhaps some code could be left unmodified,
>   implicitly truncating the value to an int.  For example I think
>   sched_pctcpu_update() is fine.  I've gotten an amd64 kernel to compile
>   and boot with this change, but it's hard to be confident in it.  This
>   approach also has the potential downside of bloating structures that
>   store a ticks value, and it can't be MFCed.
> - Introduce a 64-bit ticks variable, ticks64, and
>   #define ticks ((int)ticks64).  This requires renaming any struct
>   fields and local vars named "ticks", of which there's a decent number,
>   but that can be done fairly mechanically.
> 
> Is there another solution which avoids these pitfalls?  If not, should
> we go ahead with one of these approaches?  If so, which one?

You cannot do this in C, but can in asm:
        .data
        .globl  ticksl, ticks
        .type   ticksl, @object
        .type   ticks, @object
ticksl: .quad
        .size   ticksl, 8
ticks   =ticksl		/* for little-endian */
/* ticks	=ticksl + 4  for big-endian */
        .size   ticks, 4


Then update only ticksl in the hardclock().