[Intel-gfx] [PATCH 07/66] drm/i915: Keep the most recently used active-fence upon discard

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-07-17 13:38:01)
> 
> On 15/07/2020 12:50, Chris Wilson wrote:
> > +     /* Even if we have not used the cache, we may still have a barrier */
> > +     if (!ref->cache)
> > +             ref->cache = fetch_node(ref->tree.rb_node);
> > +
> > +     /* Keep the MRU cached node for reuse */
> > +     if (ref->cache) {
> > +             /* Discard all other nodes in the tree */
> > +             rb_erase(&ref->cache->node, &ref->tree);
> > +             root = ref->tree;
> > +
> > +             /* Rebuild the tree with only the cached node */
> > +             rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
> > +             rb_insert_color(&ref->cache->node, &ref->tree);
> > +             GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
> > +     }
> >   
> >       spin_unlock_irqrestore(&ref->tree_lock, flags);
> >   
> > @@ -156,6 +168,7 @@ __active_retire(struct i915_active *ref)
> >       /* ... except if you wait on it, you must manage your own references! */
> >       wake_up_var(ref);
> >   
> > +     /* Finally free the discarded timeline tree  */
> >       rbtree_postorder_for_each_entry_safe(it, n, &root, node) {
> >               GEM_BUG_ON(i915_active_fence_isset(&it->base));
> >               kmem_cache_free(global.slab_cache, it);
> 
> Here it frees everything.. so how does ref->cache, being in the tree, 
> survives?

This is the old root which does not contain ref->cache, as we moved that
to become the new tree.

             /* Discard all other nodes in the tree */
             rb_erase(&ref->cache->node, &ref->tree);
             root = ref->tree;