Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

Andi Kleen
2006-06-16T15:40:28Z

On Friday 16 June 2006 17:31, Zoltan Menyhart wrote:
> Andi Kleen wrote:
>
> > That is not how user space TLS works. It usually has a base a register.
>
> Can you please give me a real life (simplified) example?

On x86-64 it's just %fs:offset. gcc is a bit dumb on this and usually
loads the base address from %fs:0 first.

>
> > This means it cannot be cache colored (because you would need a static
> > offset) and you couldn't share task_structs on a page.
>
> I do not see the problem.

Your scheme relies on task_struct fields being on a known offset
in the page. But slab cache coloring varies the offset to make the data
spread out better in the caches.

> Can you explain please?
> E.g. the scheduler pulls a task instead of the current one. The CPU
> will see "current->thread_info.cpu"-s of all the tasks at the same
> offset anyway.

It varies relative to the start of page.

That was one of the bigger wins relative to the task_struct in stack
page of 2.4 had.

>
> > Also you would make task_struct part of the userland ABI which
> > seems like a very very bad idea to me. It means we couldn't change
> > it anymore.
>
> We can make some wrapper, e.g.:
>
> user_per_cpu_var(name, offset)

You would need to wrap everything and likely users would like
task_struct so much that they accessed it anyways without your wrappers.

> "vgetcpu()" would also be added to the ABI which we couldn't change
> easily either.

Yes, but it's a defined function. No different from a system call.

-Andi