Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)

On Tue, Jul 21, 2015 at 12:58:13PM +0000, Mathieu Desnoyers wrote:

----- On Jul 21, 2015, at 3:30 AM, Ondřej Bílka neleai@seznam.cz wrote:

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On Tue, Jul 21, 2015 at 12:25:00AM +0000, Mathieu Desnoyers wrote:

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Does it solve the Wine problem?  If Wine uses gs for something and
calls a function that does this, Wine still goes boom, right?

So the advantage of just making a global segment descriptor available
is that it's not *that* expensive to just save/restore segments. So
either wine could do it, or any library users would do it.

But anyway, I'm not sure this is a good idea. The advantage of it is
that the kernel support really is _very_ minimal.

Considering that we'd at least also want this feature on ARM and
PowerPC 32/64, and that the gs segment selector approach clashes with
existing apps (wine), I'm not sure that implementing a gs segment
selector based approach to cpu number caching would lead to an overall
decrease in complexity if it leads to performance similar to those of
portable approaches.

I'm perfectly fine with architecture-specific tweaks that lead to
fast-path speedups, but if we have to bite the bullet and implement
an approach based on TLS and registering a memory area at thread start
through a system call on other architectures anyway, it might end up
being less complex to add a new system call on x86 too, especially if
fast path overhead is similar.

But I'm inclined to think that some aspect of the question eludes me,
especially given the amount of interest generated by the gs-segment
selector approach. What am I missing ?

As I wrote before you don't have to bite bullet as I said before. It
suffices to create 128k element array with cpu for each tid, make that
mmapable file and userspace could get cpu with nearly same performance
without hacks.

I don't see how this would be acceptable on memory-constrained embedded
systems. They have multiple cores, and performance requirements, so
having a fast getcpu would be useful there (e.g. telecom industry),
but they clearly cannot afford a 512kB table per process just for that.

Which just means that you need more complicated api and implementation
for that but idea stays same. You would need syscalls
register/deregister_cpuid_idx that would give you index used instead
tid. A kernel would need to handle that many ids could be registered for
each thread and resize mmaped file in syscalls.