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Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()
- From: Andi Kleen <ak at suse dot de>
- To: Zoltan Menyhart <Zoltan dot Menyhart at bull dot net>
- Cc: Jes Sorensen <jes at sgi dot com>, Tony Luck <tony dot luck at intel dot com>, discuss at x86-64 dot org, linux-kernel at vger dot kernel dot org, libc-alpha at sourceware dot org, vojtech at suse dot cz, linux-ia64 at vger dot kernel dot org
- Date: Fri, 16 Jun 2006 17:37:06 +0200
- Subject: Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()
- References: <200606140942.31150.ak@suse.de> <200606161656.40930.ak@suse.de> <4492CEC0.2080102@bull.net>
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