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

["Ondřej Bílka" <neleai-9Vj9tDbzfuSlVyrhU4qvOw@public.gmane.org>]

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