Messages in this thread | | | From | Andi Kleen <> | Subject | Re: FOR REVIEW: New x86-64 vsyscall vgetcpu() | Date | Fri, 16 Jun 2006 17:37:06 +0200 |
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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
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