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

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

On Fri, Jul 17, 2015 at 04:03:30PM +0000, Mathieu Desnoyers wrote:
> ----- On Jul 17, 2015, at 6:58 AM, Ondřej Bílka neleai@seznam.cz wrote:
> [...]
> >> If we invoke this per-thread registration directly in the glibc NPTL
> >> implementation,
> >> in start_thread, do you think it would fit your requirements ?
> >>
> > A generic solution would be adding eager initialization of thread_local
> > variables which would fix more performance problems.
> 
> Concretely, what code would be in charge of this initialization ?
> 
> What changes does "eager initialization" imply from the current
> implementation ?
>
Now if you write

class foo{
public:
  foo(){
    printf("init\n");
  }
  foo(const foo &x){
    printf("init\n");
  }
  int bar()
  {
    return 32;
  }
};
thread_local class foo x;

Then constructor isn't called in all threads, only main as its lazily
initialized. You need to call x.bar() in each thread to cause
initialization.

> > Second would be write patch to libc adding function
> > pthread_create_add_hook_np to register function that would be ran after
> > each thread creation.
> 
> Do you suggest that this callback should be registered once per thread,
> or somehow attached to pthread_attr_t ? As maintainer of lttng-ust,
> where we need some control over our own threads, but where we want minimal
> impact on the overall application, I would really prefer if we can attach
> the callback to pthread_attr_t.
> 
Could you elaborate. I was suggesting per-thread callback, what do you
want to do with pthread_attr_t?