Eric Dumazet wrote:
On Wed, 04 Apr 2007 18:55:18 +1000
Nick Piggin <[EMAIL PROTECTED]> wrote:
Peter Zijlstra wrote:
On Wed, 2007-04-04 at 12:22 +1000, Nick Piggin wrote:
Eric Dumazet wrote:
I do think such workloads might benefit from a vma_cache not shared by
all threads but private to each thread. A sequence could invalidate the
cache(s).
ie instead of a mm->mmap_cache, having a mm->sequence, and each thread
having a current->mmap_cache and current->mm_sequence
I have a patchset to do exactly this, btw.
/me too
However, I decided against pushing it because when it does happen that a
task is not involved with a vma lookup for longer than it takes the seq
count to wrap we have a stale pointer...
We could go and walk the tasks once in a while to reset the pointer, but
it all got a tad involved.
Well here is my core patch (against I think 2.6.16 + a set of vma cache
cleanups and abstractions). I didn't think the wrapping aspect was
terribly involved.
Well, I believe this one is too expensive. I was thinking of a light one :
I am not deleting mmap_sem, but adding a sequence number to mm_struct, that is
incremented each time a vma is added/deleted, not each time mmap_sem is taken
(read or write)
That's exactly what mine does (except IIRC it doesn't invalidate when
you add a vma).
Each thread has its own copy of the sequence, taken at the time find_vma() had
to do a full lookup.
I believe some optimized paths could call check_vma_cache() without mmap_sem
read lock taken, and if it fails, take the mmap_sem lock and do the slow path.
The mmap_sem for read does not only protect the mm_rb rbtree structure, but
the vmas themselves as well as their page tables, so you can't do that.
You could do it if you had a lock-per-vma to synchronise against write
operations, and rcu-freed vmas or some such... but I don't think we should
go down a road like that until we first remove mmap_sem from low hanging
things (like private futexes!) and then see who's complaining.
--
SUSE Labs, Novell Inc.
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