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

On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote:

On Thu, Jul 16, 2015 at 12:27 PM, Andy Lutomirski [off-list ref] wrote:

quoted

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.