Re: widening ticks

On Thu, Jan 09, 2025 at 12:18:48AM +0200, Konstantin Belousov wrote:
> 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().

Oh, that's better than my proposals.  I guess it could be done in the
kernel linker script too, but perhaps locore.S is a better place anyway.