[Intel-gfx] [PATCH v2 1/3] drm/i915: Enable lockless lookup of request tracking via RCU

[Peter Zijlstra]

On Wed, Jan 06, 2016 at 09:06:58AM +0100, Daniel Vetter wrote:
> This pretty much went over my head ;-) What I naively hoped for is that
> kfree() on an rcu-freeing slab could be tought to magically stall a bit
> (or at least expedite the delayed freeing) if we're piling up too many
> freed objects.

Well, RCU does try harder when the callback list is getting 'big' (10k
IIRC).

> Doing that only in OOM is probably too late since OOM
> handling is a bit unreliable/unpredictable. And I thought we're not the
> first ones running into this problem.

The whole memory pressure thing is unreliable/unpredictable last time I
looked at it, but sure, I suppose we could try and poke RCU sooner, but
then you get into the problem of when, doing it too soon will be
detrimental to performance, doing it too late is, well, too late.

> Do all the other users of rcu-freed slabs just open-code their own custom
> approach? If that's the recommendation we can certainly follow that, too.

The ones I know of seem to simply ignore this problem..