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

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Jan 27, 2016 at 06:43:36PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 1:03 PM, Josh Triplett josh@joshtriplett.org wrote:
> 
> > On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:
> >> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@linutronix.de 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.
> 
> I'm referring to the code path that read the content of the cache.
> This code don't call the getcpu_cache system call each time (this
> would defeat the entire purpose of this cache), but still has to
> know whether it can rely on the cache content to contain the current
> CPU number. Seeing a "-1" there is a nice way to tell the fast path
> that it needs to go through a fallback.
> 
> Or perhaps you have another mechanism in mind for that ? How do
> you intend to communicate the ENOSYS from the kernel to all
> eventual readers of the cache, without adding extra function
> call overhead on the fast path ?

Have the fast path assume the cache, without even checking for -1; only
use that fast path if getcpu_cache exists.  If you don't have
getcpu_cache, don't even attempt to use the fast path; substitute in a
fallback implementation.  Don't have a conditional in either version;
just decide which version to use based on system capabilities.

Alternatively, use the implementation you have with a placeholder value,
and just use 0xFFFFFFFF as the placeholder; that seems no more or
less valid.