On 04/07/2018 12:47, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2018-07-04 12:34:04)
On 04/07/2018 10:39, Tvrtko Ursulin wrote:
On 04/07/2018 09:34, Chris Wilson wrote:
Using a VMA on more than one timeline concurrently is the exception
rather than the rule (using it concurrently on multiple engines). As we
expect to only use one active tracker, store the most recently used
tracker inside the i915_vma itself and only fallback to the rbtree if
we need a second or more concurrent active trackers.
v2: Comments on how we overwrite any existing last_active cache.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_vma.c | 50 +++++++++++++++++++++++++++++++--
drivers/gpu/drm/i915/i915_vma.h | 1 +
2 files changed, 49 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_vma.c
b/drivers/gpu/drm/i915/i915_vma.c
index cd94ffc7f079..33925e00f7e8 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -119,6 +119,12 @@ i915_vma_retire(struct i915_gem_active *base,
struct i915_request *rq)
__i915_vma_retire(active->vma, rq);
}
+static void
+i915_vma_last_retire(struct i915_gem_active *base, struct
i915_request *rq)
+{
+ __i915_vma_retire(container_of(base, struct i915_vma,
last_active), rq);
+}
+
static struct i915_vma *
vma_create(struct drm_i915_gem_object *obj,
struct i915_address_space *vm,
@@ -136,6 +142,7 @@ vma_create(struct drm_i915_gem_object *obj,
vma->active = RB_ROOT;
+ init_request_active(&vma->last_active, i915_vma_last_retire);
init_request_active(&vma->last_fence, NULL);
vma->vm = vm;
vma->ops = &vm->vma_ops;
@@ -895,6 +902,22 @@ static struct i915_gem_active
*lookup_active(struct i915_vma *vma, u64 idx)
{
struct i915_vma_active *active;
struct rb_node **p, *parent;
+ struct i915_request *old;
+
+ /*
+ * We track the most recently used timeline to skip a rbtree search
+ * for the common case, under typical loads we never need the rbtree
+ * at all. We can reuse the last_active slot if it is empty, that is
+ * after the previous activity has been retired, or if the active
+ * matches the current timeline.
+ */
+ old = i915_gem_active_raw(&vma->last_active,
+ &vma->vm->i915->drm.struct_mutex);
+ if (!old || old->fence.context == idx)
+ goto out;
+
+ /* Move the currently active fence into the rbtree */
+ idx = old->fence.context;
parent = NULL;
p = &vma->active.rb_node;
@@ -903,7 +926,7 @@ static struct i915_gem_active
*lookup_active(struct i915_vma *vma, u64 idx)
active = rb_entry(parent, struct i915_vma_active, node);
if (active->timeline == idx)
- return &active->base;
+ goto replace;
if (active->timeline < idx)
p = &parent->rb_right;
@@ -922,7 +945,25 @@ static struct i915_gem_active
*lookup_active(struct i915_vma *vma, u64 idx)
rb_link_node(&active->node, parent, p);
rb_insert_color(&active->node, &vma->active);
- return &active->base;
+replace:
+ /*
+ * Overwrite the previous active slot in the rbtree with
last_active,
+ * leaving last_active zeroed. If the previous slot is still active,
+ * we must be careful as we now only expect to recieve one retire
typo in receive
+ * callback not two, and so much undo the active counting for the
+ * overwritten slot.
+ */
+ if (i915_gem_active_isset(&active->base)) {
+ __list_del_entry(&active->base.link);
+ vma->active_count--;
> + GEM_BUG_ON(!vma->active_count);
I still don't get this. The cache is exclusive, so when transferring a
record from rbtree to last_active, why do we need to decrement the
vma->active_count here? Don't get the part in the comment about two
retires - do you really sometimes expect two - ie cache is not exclusive?
But the fact that lookup of a cached entry is a straight return, meaning
vma->active_count is manipulated elsewhere, makes me think it is
avoidable messing with it on this path as well.
Maybe the separation of duties between the callers and this function
needs to be stronger.
Hmm or is your cache actually inclusive? Don't see no rbtree
manipulation on migration to and from last_active/rbtree..
Both. Inclusive in the sense that both last_active and its timeline slot
in the rbtree may be active tracking different requests and so receive
retirement callbacks independently. Exclusive in that we don't store
last_active in the cache slot and in the rbtree.
I don't know how to reconcile between two sentences (statements). They
seem in contradiction. :(
>> And since rbtree lookup is always for the last_active context id, you
would otherwise never hit the the "goto replace" path.
How do you ever look up an id which is not cached in last_active then?
We don't. We only lookup on evicting a still active request from
last_active. The MRU recent request always goes into last_active.
Sorry lost, another day maybe.
Regards,
Tvrtko
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