Re: [PATCHv2 1/2] arm64:vdso: Rewrite gettimeofday into C.

[Will Deacon]

Hi Andrew,

Thanks for posting this, but please try to cc the maintainers in future -- I
almost missed it!

On Tue, May 30, 2017 at 05:34:19PM -0700, Andrew Pinski wrote:
> This allows the compiler to optimize the divide by 1000.
> And remove the other divide.
> 
> On ThunderX, gettimeofday improves by 32%.  On ThunderX 2,
> gettimeofday improves by 18%.
> 
> Note I noticed a bug in the old implementation of __kernel_clock_getres;
> it was checking only the lower 32bits of the pointer; this would work
> for most cases but could fail in a few.
> 
> Changes from v1:
> * Fixed bug in __kernel_clock_getres for checking the pointer argument.
> * Fix comments to refer to functions in arm64.

I tested this patch on a few platforms I have access to and didn't see the
perf regressions I saw when I looked at this in the past with an older
toolchain (it was mostly about the same, with a couple of improvements).

So, in principle, I'm not opposed to moving this into C. However, we're
currently close to a "vDSO-explosion" on arm64 with people wanting a compat
variant and also an ILP32 variant. When Kevin posted his compat variant
(also in C):

  http://lkml.kernel.org/r/20161206160353.14581-1-kevin.brodsky@xxxxxxx

Nathan (who apparently needs to set his mail host address ;) was concerned
about duplication between arm and arm64:

  http://lkml.kernel.org/r/87r35jmv3e.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I'm firmly of the opinion that we should try to write an arch-agnostic vDSO
implementation in core code (lib/vdso or something) where the arch header
provides things like:

  * The mechanism to read the counter
  * The mechanism to issue a syscall
  * A function to determine whether or not the current clocksource is
    suitable

I think the datapage format could be defined in core code and it would be
worth looking to see how much the virtual mapping code can be consolidated
too.

If we can get something that works for arm native, arm64 native, arm64
compat and arm64 ilp32 then it's probably going to be useful for other
architectures too, even if we need to add more customisation points in
future.

I've spoken to Kevin about this, but I'm not sure whether he's had a chance
to look at knocking up a prototype. A first stab could just unconditionally
fallback to the system call.

Will