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

[OndÅej BÃlka <neleai at seznam dot cz>]

On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote:
> On Thu, Jul 16, 2015 at 12:27 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > If we actually bit the bullet and implemented per-cpu mappings
> 
> That's not ever going to happen.
> 
> The Linux VM model of "one page table per VM" is the right one.
> Anything else sucks, and makes threading a disaster.
> 
> So you can try to prove me wrong, but seriously, I doubt you'll succeed.
> 
> On x86, if you want per-cpu memory areas, you should basically plan on
> using segment registers instead (although other odd state has been
> used - there's been the people who use segment limits etc rather than
> the *pointer* itself, preferring to use "lsl" to get percpu data. You
> could also imaging hiding things in the vector state somewhere if you
> control your environment well enough).
>
Thats correct, problem is that you need some sort of hack like this on
archs that otherwise would need syscall to get tid/access tls variable.

On x64 and archs that have register for tls this could be implemented
relatively easily.

Kernel needs to allocate 

int running_cpu_for_tid[32768];

On context switch it atomically writes to this table 

running_cpu_for_tid[tid] = cpu;

This table is read-only accessible from userspace as mmaped file.

Then userspace just needs to access it with three indirections like:

__thread tid;

char caches[CPU_MAX];
#define getcpu_cache caches[tid > 32768 ? get_cpu() : running_cpu_for_tid[tid]]
 
With more complicated kernel interface you could eliminate one
indirection as we would use void * array instead and thread could do
syscall to register what values it should use for each thread.