* Linus Torvalds [off-list ref] wrote:

On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar [off-list ref] wrote:

quoted

So as a quick concept hack I wrote the patch attached below.
(It's not signed off, see the patch description text for the
reason.)

Well, it confirms that anon_vma locking is a big problem, but 
as outlined in my other email it's completely incorrect from 
an actual behavior standpoint.

Yeah.

Btw, I think the anon_vma lock could be made a spinlock 
instead of a mutex or rwsem, but that would probably take more 
work. We *shouldn't* be doing anything that needs IO inside 
the anon_vma lock, though, so it *should* be doable. But there 
are probably quite a bit of allocations inside the lock, and I 
know it covers huge areas, so a spinlock might not only be 
hard to convert to, it quite likely has latency issues too.

I'll try the rwsem and see how it goes?

Oh, btw, MUTEX_SPIN_ON_OWNER may well improve performance too, 
but it gets disabled by DEBUG_MUTEXES. So some of the 
performance impact of the vma locking may be *very* 
kernel-config dependent.

Hm, indeed. For performance runs I typically disable lock 
debugging - which might have made me not directly notice some of 
the performance problems.

Thanks,

	Ingo

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