Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

[Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>]

On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> > On Wed, 27 Jan 2016, Josh Triplett wrote:
> >> With the dynamic allocation removed, this seems sensible to me.  One
> >> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU
> >> number should never need to hold a negative number.
> > 
> > You try to block the future of computing: https://lwn.net/Articles/638673/
> 
> Besides impossible architectures, there is actually a use-case for
> signedness here. It makes it possible to initialize the cpu number
> cache to a negative value, e.g. -1, in userspace. Then, a check for
> value < 0 can be used to figure out cases where the getcpu_cache
> system call is not implemented, and where a fallback (vdso or getcpu
> syscall) needs to be used.
> 
> This is why I have chosen a signed type for the cpu cache so far.

If getcpu_cache doesn't exist, you'll get ENOSYS.  If getcpu_cache
returns 0, then you can assume the kernel will give you a valid CPU
number.

- Josh Triplett