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

[Tvrtko Ursulin]

On 15/07/2020 12:50, Chris Wilson wrote:
Whenever an i915_active idles, we prune its tree of old fence slots to
prevent a gradual leak should it be used to track many, many timelines.
The downside is that we then have to frequently reallocate the rbtree.
A compromise is that we keep the most recently used fence slot, and
reuse that for the next active reference as that is the most likely
timeline to be reused.
>
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
---
  drivers/gpu/drm/i915/i915_active.c | 27 ++++++++++++++++++++-------
  drivers/gpu/drm/i915/i915_active.h |  4 ----
  2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c 
b/drivers/gpu/drm/i915/i915_active.c
index 799282fb1bb9..0854b1552bc1 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -130,8 +130,8 @@ static inline void debug_active_assert(struct i915_active 
*ref) { }
  static void
  __active_retire(struct i915_active *ref)
  {
+       struct rb_root root = RB_ROOT;
        struct active_node *it, *n;
-       struct rb_root root;
        unsigned long flags;
  
  	GEM_BUG_ON(i915_active_is_idle(ref));
@@ -143,9 +143,21 @@ __active_retire(struct i915_active *ref)
        GEM_BUG_ON(rcu_access_pointer(ref->excl.fence));
        debug_active_deactivate(ref);
  
-	root = ref->tree;
-       ref->tree = RB_ROOT;
-       ref->cache = NULL;
+       /* 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?

@@ -750,16 +763,16 @@ int i915_sw_fence_await_active(struct i915_sw_fence 
*fence,
        return await_active(ref, flags, sw_await_fence, fence, fence);
  }
  
-#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
  void i915_active_fini(struct i915_active *ref)
  {
        debug_active_fini(ref);
        GEM_BUG_ON(atomic_read(&ref->count));
        GEM_BUG_ON(work_pending(&ref->work));
-       GEM_BUG_ON(!RB_EMPTY_ROOT(&ref->tree));
        mutex_destroy(&ref->mutex);
+
+       if (ref->cache)
+               kmem_cache_free(global.slab_cache, ref->cache);
  }
-#endif
  
  static inline bool is_idle_barrier(struct active_node *node, u64 idx)
  {
diff --git a/drivers/gpu/drm/i915/i915_active.h 
b/drivers/gpu/drm/i915/i915_active.h
index 73ded3c52a04..b9e0394e2975 100644
--- a/drivers/gpu/drm/i915/i915_active.h
+++ b/drivers/gpu/drm/i915/i915_active.h
@@ -217,11 +217,7 @@ i915_active_is_idle(const struct i915_active *ref)
        return !atomic_read(&ref->count);
  }
  
-#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
  void i915_active_fini(struct i915_active *ref);
-#else
-static inline void i915_active_fini(struct i915_active *ref) { }
-#endif
  
  int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
                                            struct intel_engine_cs *engine);

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