Re: RCU red-black tree (was: Re: [PATCH 4/6] kvm tools: Add rwlock wrapper)

Mathieu Desnoyers
2011-05-30T11:18:18Z

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Sun, May 29, 2011 at 01:01:04PM -0400, Mathieu Desnoyers wrote:
> > * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> > > * Sasha Levin (levinsasha928@gmail.com) wrote:
> > [...]
> > > > Hi Mathieu!
> > > >
> > > > In tools/kvm/ we use a rb-tree (same one used by the kernel) with the
> > > > augmentation feature to support an interval rb-tree - which means that
> > > > every update to the tree not only updates the nodes directly related to
> > > > the updated node but also all the nodes on the path to the root of the
> > > > tree.
> > >
> > > Cool !!
> > >
> > > I'm adding in copy Phil Howard who has been working on RCU RB tree for
> > > much longer than myself.
> > >
> > > > I see that in liburcu there is an implementation of a rcu linked list
> > > > but no implementation of a rb-tree.
> > > >
> > > > Are you currently working on one? or maybe I should try writing one and
> > > > sending it to you?
> > >
> > > Actually, I started working on one last year, but had to interrupt my
> > > effort before I got it even working right.
> > [...]
> > > We'd have to see how we can go from this implementation of a standard RB
> > > tree to an interval RB tree too. I guess it will depend whether you need
> > > the updates from the target node up to the root to be done "all at once"
> > > from a reader perspective (then you would probably need to replace a
> > > copy of a part of the tree all at once), or if you can allow the update
> > > to be done piece-wise on a node-by-node basis as readers go through the
> > > tree (from root to leafs).
> >
> > I've revisited the RCU rbtree implementation this weekend, and it works
> > much better now. I reimplemented the whole thing from 0 starting from
> > the CLRS chapter 12 algorithms (to get the non-rcu
> > (insertion/removal)-only stress-tests working) and incrementally
> > RCU-ized the updates and then added read-side tests. All along, I used
> > the test_urcu_rbtree test case that does some basic coherency tests by
> > searching for some random elements that *should* be there in parellel
> > with insertion and removals. The implementation I currently have
> > survives the "search for known elements in parallel with updates" stress
> > test (so far). (e.g. test_urcu_rbtree 6 2 10 -g 30 : 6 readers, 2
> > writers, 30 known random elements, writers are adding/removing 6 random
> > elements, on a 8-core machine)
> >
> > See: git://git.lttng.org/userspace-rcu.git
> > branch : rbtree2
> >
> > The key idea I used in this implementation is to "decay" the old nodes
> > (AFAIK, I just made this up) : "decaying" a node could be best described
> > as creating an exact copy of a node, and putting a pointer to this new
> > node into the old node to form a "decay chain". This allowed me to keep
> > the algorithm very much similar to CLRS by just walking the decay chains
> > whenever needed. The old node "decays" by using call_rcu to free it
> > after a grace period passes. This imply that the updates must hold the
> > RCU read-side lock in addition to a mutex to make sure the decaying
> > nodes stay valid for the duration of their use.
> >
> > This implementation never requires the read-side to loop, thus
> > guaranteeing a wait-free read-side behavior (so search operations will
> > always be strictly log(n) without any busy-loop delay).
> >
> > I have not created stress-tests for next/prev walk of the tree yet. It
> > is therefore entirely possible that this does not work as expected.
> >
> > Comments are welcome,
>
> Very cool!
>
> The trick Phil Howard used allowed him to avoid duplicating the nodes
> in some cases in the rotations. I might be missing something, but it
> looks like you are duplicating in all cases.

The duplications I do are (following CLRS 3rd ed. chap 12, 13):

- x, y and beta for left and right rotation (p. 313)
- v for transplant (p. 296)
- the whole branch between z.right and y (inclusive) for lines 9--20 of
rb_delete() (p. 324, chap. 13) (at most log(n) items), for the case I
call rcu_rbtree_remove_nonil() in my code.

> Would using Phil's trick
> result in significant performance gain?

I just read through Phil's paper at
http://www.cs.pdx.edu/pdfs/tr1006.pdf. It looks like we have different
targets: Phil's structure of RB tree is heavily tuned to allow RCU
search, but it uses a RW lock for in-order traversal. Mine allows both
search and in-order traversal to be performed under RCU read-side.

One impact of my different goal is that I need to keep pointers to
parent nodes (and must know if a node is a left or right child) -- and
update both of these atomically. E.g., at least one optimisation done in
Phil's work would not work with my scheme (his optimized swap, 4.1.2):
it generates an intermediate tree state where in-order traversal could
loop between C -> B -> A -> C (trying to do multiple rcu_rbtree_next)
for a while which goes against the time guarantees I want to provide.

Thanks,

Mathieu

>
> Thanx, Paul
>
> > Thanks,
> >
> > Mathieu
> >
> >
> > >
> > > Thanks,
> > >
> > > Mathieu
> > >
> > >
> > > --
> > > Mathieu Desnoyers
> > > Operating System Efficiency R&D Consultant
> > > EfficiOS Inc.
> > > http://www.efficios.com
> >
> > --
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.