Re: [RFC][PATCH 5/9] rbtree: Make lockless searches non-fatal

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Mar 01, 2015 at 01:11:23PM -0800, Michel Lespinasse wrote:
> On Sat, Feb 28, 2015 at 1:24 PM, Peter Zijlstra <peterz@infradead.org> wrote:

> > It generates slightly worse code, probably because gcc stinks at
> > volatile. But in pointer chasing heavy code a few instructions more
> > should not matter.
> 
> So, I was worried that this would penalize all rbtree users, for the
> benefit of just the one you're adding later in this series. We have
> several rbtrees where we care about performance a lot, such as the
> ones used in the scheduler or for indexing vmas.
> 
> That said, I checked with the compiler we are using here (gcc 4.7
> variant) and I didn't see any change in the generated code. So, no
> issue here for me.
> 
> If the object code really is different in your setup, please use the
> lib/rbtree_test module to check the performance impact of the change.

I can do that; I had similar results to what Ingo posted. I meant to go
build a 4.9 or 5.0 compiler to see what current GCC makes of it, but
I've not yet gotten around to doing so.

My result were with 4.8.3 iirc.

> > For 2) I have carefully audited the code and drawn every intermediate
> > link state and not found a loop.
> 
> As Mathieu remarked, we are never modifying the currently active tree,
> so the interrupt case is not the reason for avoiding loops.

Correct, for the proposed use we do no. I did however double (actually
triple) check this property because I feel its a good property to have,
no matter what you do to the tree, a (simple) lookup will be non-fatal.

But yes, I'll clarify things.

> I think your proposal will work well for the use case you have in mind
> (looking up modules based on address). However, I was wondering how
> you would compare your proposal against an alternative I hard Josh
> Triplett formulate before, where there would be one unique rbtree but
> rotations would allocate new nodes rather than modify the existing
> ones. I think this would be workable as well; I'm just not sure
> whether this would be more or less generally applicable than your
> proposal. Copying Josh in case he wants to chime in.

So I was not aware of that particular solution.

It changes the rb-tree from using internal storage like we do now, to
requiring external storage.

I do have experience with making an RCU safe (in memory) B+tree, and
there the allocations were absolutely killing performance.