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

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:

quoted

On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:

quoted

----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@linutronix.de wrote:

quoted

On Wed, 27 Jan 2016, Josh Triplett wrote:

quoted

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.