On 14/10/2016 13:18, Chris Wilson wrote:
A while ago we switched from a contiguous array of pages into an sglist,
for that was both more convenient for mapping to hardware and avoided
the requirement for a vmalloc array of pages on every object. However,
certain GEM API calls (like pwrite, pread as well as performing
relocations) do desire access to individual struct pages. A quick hack
was to introduce a cache of the last access such that finding the
following page was quick - this works so long as the caller desired
sequential access. Walking backwards, or multiple callers, still hits a
slow linear search for each page. One solution is to store each
successful lookup in a radix tree.
v2: Rewrite building the radixtree for clarity, hopefully.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_drv.h | 69 +++++-------
drivers/gpu/drm/i915/i915_gem.c | 179 +++++++++++++++++++++++++++++---
drivers/gpu/drm/i915/i915_gem_stolen.c | 4 +-
drivers/gpu/drm/i915/i915_gem_userptr.c | 4 +-
4 files changed, 193 insertions(+), 63 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 38467dde1efe..53cf4b0e5359 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2273,9 +2273,12 @@ struct drm_i915_gem_object {
struct sg_table *pages;
int pages_pin_count;
- struct get_page {
- struct scatterlist *sg;
- int last;
+ struct i915_gem_object_page_iter {
+ struct scatterlist *sg_pos;
+ unsigned long sg_idx;
We are not consistent in the code with type used for number of pages in
an object. sg_alloc_table even takes an unsigned int for it, so for as
long as we build them as we do, unsigned long is a waste.
+
+ struct radix_tree_root radix;
+ spinlock_t lock;
} get_page;
void *mapping;
@@ -2478,6 +2481,14 @@ static __always_inline struct sgt_iter {
return s;
}
+static inline struct scatterlist *____sg_next(struct scatterlist *sg)
+{
+ ++sg;
+ if (unlikely(sg_is_chain(sg)))
+ sg = sg_chain_ptr(sg);
+ return sg;
+}
+
/**
* __sg_next - return the next scatterlist entry in a list
* @sg: The current sg entry
@@ -2492,9 +2503,7 @@ static inline struct scatterlist *__sg_next(struct scatterlist *sg)
#ifdef CONFIG_DEBUG_SG
BUG_ON(sg->sg_magic != SG_MAGIC);
#endif
- return sg_is_last(sg) ? NULL :
- likely(!sg_is_chain(++sg)) ? sg :
- sg_chain_ptr(sg);
+ return sg_is_last(sg) ? NULL : ____sg_next(sg);
}
/**
@@ -3170,45 +3179,21 @@ static inline int __sg_page_count(struct scatterlist *sg)
return sg->length >> PAGE_SHIFT;
}
-struct page *
-i915_gem_object_get_dirty_page(struct drm_i915_gem_object *obj, int n);
-
-static inline dma_addr_t
-i915_gem_object_get_dma_address(struct drm_i915_gem_object *obj, int n)
-{
- if (n < obj->get_page.last) {
- obj->get_page.sg = obj->pages->sgl;
- obj->get_page.last = 0;
- }
-
- while (obj->get_page.last + __sg_page_count(obj->get_page.sg) <= n) {
- obj->get_page.last += __sg_page_count(obj->get_page.sg++);
- if (unlikely(sg_is_chain(obj->get_page.sg)))
- obj->get_page.sg = sg_chain_ptr(obj->get_page.sg);
- }
-
- return sg_dma_address(obj->get_page.sg) + ((n - obj->get_page.last) << PAGE_SHIFT);
-}
-
-static inline struct page *
-i915_gem_object_get_page(struct drm_i915_gem_object *obj, int n)
-{
- if (WARN_ON(n >= obj->base.size >> PAGE_SHIFT))
- return NULL;
+struct scatterlist *
+i915_gem_object_get_sg(struct drm_i915_gem_object *obj,
+ unsigned long n, unsigned int *offset);
- if (n < obj->get_page.last) {
- obj->get_page.sg = obj->pages->sgl;
- obj->get_page.last = 0;
- }
+struct page *
+i915_gem_object_get_page(struct drm_i915_gem_object *obj,
+ unsigned long n);
- while (obj->get_page.last + __sg_page_count(obj->get_page.sg) <= n) {
- obj->get_page.last += __sg_page_count(obj->get_page.sg++);
- if (unlikely(sg_is_chain(obj->get_page.sg)))
- obj->get_page.sg = sg_chain_ptr(obj->get_page.sg);
- }
+struct page *
+i915_gem_object_get_dirty_page(struct drm_i915_gem_object *obj,
+ unsigned long n);
- return nth_page(sg_page(obj->get_page.sg), n - obj->get_page.last);
-}
+dma_addr_t
+i915_gem_object_get_dma_address(struct drm_i915_gem_object *obj,
+ unsigned long n);
static inline void i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
{
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 95781c63f605..d736beb4d57f 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2333,6 +2333,15 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj)
kfree(obj->pages);
}
+static void __i915_gem_object_reset_page_iter(struct drm_i915_gem_object *obj)
+{
+ struct radix_tree_iter iter;
+ void **slot;
+
+ radix_tree_for_each_slot(slot, &obj->get_page.radix, &iter, 0)
+ radix_tree_delete(&obj->get_page.radix, iter.index);
+}
+
int
i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
{
@@ -2365,6 +2374,8 @@ i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
obj->mapping = NULL;
}
+ __i915_gem_object_reset_page_iter(obj);
+
ops->put_pages(obj);
obj->pages = NULL;
@@ -2534,8 +2545,8 @@ i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
list_add_tail(&obj->global_list, &dev_priv->mm.unbound_list);
- obj->get_page.sg = obj->pages->sgl;
- obj->get_page.last = 0;
+ obj->get_page.sg_pos = obj->pages->sgl;
+ obj->get_page.sg_idx = 0;
return 0;
}
@@ -4338,6 +4349,8 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
obj->frontbuffer_ggtt_origin = ORIGIN_GTT;
obj->madv = I915_MADV_WILLNEED;
+ INIT_RADIX_TREE(&obj->get_page.radix, GFP_ATOMIC | __GFP_NOWARN);
Pros & cons of GFP_ATOMIC here? Versus perhaps going with the mutex? I
don't know how much data radix tree allocates with this, per node, but
we can have a lot of pages. Would this create extra pressure on slab
shrinking, and in turn out objects?
+ spin_lock_init(&obj->get_page.lock);
i915_gem_info_add_obj(to_i915(obj->base.dev), obj->base.size);
}
@@ -5016,21 +5029,6 @@ void i915_gem_track_fb(struct drm_i915_gem_object *old,
}
}
-/* Like i915_gem_object_get_page(), but mark the returned page dirty */
-struct page *
-i915_gem_object_get_dirty_page(struct drm_i915_gem_object *obj, int n)
-{
- struct page *page;
-
- /* Only default objects have per-page dirty tracking */
- if (WARN_ON(!i915_gem_object_has_struct_page(obj)))
- return NULL;
-
- page = i915_gem_object_get_page(obj, n);
- set_page_dirty(page);
- return page;
-}
-
/* Allocate a new GEM object and fill it with the supplied data */
struct drm_i915_gem_object *
i915_gem_object_create_from_data(struct drm_device *dev,
@@ -5071,3 +5069,150 @@ fail:
i915_gem_object_put(obj);
return ERR_PTR(ret);
}
+
+struct scatterlist *
+i915_gem_object_get_sg(struct drm_i915_gem_object *obj,
+ unsigned long n,
Same unsigned long vs unsigned int as above.
+ unsigned int *offset)
+{
+ struct i915_gem_object_page_iter *iter = &obj->get_page;
+ struct scatterlist *sg;
+ unsigned long idx, count;
+
+ GEM_BUG_ON(n >= obj->base.size >> PAGE_SHIFT);
+ GEM_BUG_ON(obj->pages_pin_count == 0);
+
+ /* As we iterate forward through the sg, we record each entry in a
+ * radixtree for quick repeated (backwards) lookups. If we have seen
+ * this index previously, we will have an entry for it.
+ *
+ * Initial lookup is O(N), but this is amortized to O(1) for
+ * sequential page access (where each new request is consecutive
+ * to the previous one). Repeated lookups are O(lg(obj->base.size)),
+ * i.e. O(1) with a large constant!
+ */
+ if (n < READ_ONCE(iter->sg_idx))
+ goto lookup;
+
+ spin_lock(&iter->lock);
+
+ sg = iter->sg_pos;
+ idx = iter->sg_idx;
+ count = __sg_page_count(sg);
+
+ while (idx + count < n) {
+ unsigned long exception, i;
+ int ret;
+
+ /* If we cannot allocate and insert this entry, or the
+ * individual pages from this range, cancel updating the
+ * sg_idx so that on this lookup we are forced to linearly
+ * scan onwards, but on future lookups we will try the
+ * insertion again (in which case we need to be careful of
+ * the error return reporting that we have already inserted
+ * this index).
+ */
+ ret = radix_tree_insert(&iter->radix, idx, sg);
+ if (ret && ret != -EEXIST)
+ goto scan;
What other error can we get here? Internal allocation failure?
+
+ exception =
+ RADIX_TREE_EXCEPTIONAL_ENTRY |
+ idx << RADIX_TREE_EXCEPTIONAL_SHIFT;
+ for (i = 1; i < count; i++) {
+ ret = radix_tree_insert(&iter->radix, idx + i,
+ (void *)exception);
+ if (ret && ret != -EEXIST)
+ goto scan;
+ }
+
+ idx += count;
+ sg = ____sg_next(sg);
+ count = __sg_page_count(sg);
+ }
+
+scan:
+ iter->sg_pos = sg;
+ iter->sg_idx = idx;
+
+ spin_unlock(&iter->lock);
+
+ if (unlikely(n < idx)) /* insertion completed by another thread */
+ goto lookup;
+
+ /* In case we failed to insert the entry into the radixtree, we need
+ * to look beyond the current sg.
+ */
+ while (idx + count <= n) {
+ idx += count;
+ sg = ____sg_next(sg);
+ count = __sg_page_count(sg);
+ }
+
Hmm... I assume failures happen then since you added this fallback. Due
GFP_ATOMIC?
+ *offset = n - idx;
+ return sg;
+
+lookup:
+ rcu_read_lock();
+
Property of the radix tree implementation that the RCU lock is sufficient?
+ sg = radix_tree_lookup(&iter->radix, n);
+ GEM_BUG_ON(!sg);
+
+ /* If this index is in the middle of multi-page sg entry,
+ * the radixtree will contain an exceptional entry that points
+ * to the start of that range. We will return the pointer to
+ * the base page and the offset of this page within the
+ * sg entry's range.
+ */
+ *offset = 0;
+ if (unlikely(radix_tree_exception(sg))) {
+ unsigned long base =
+ (unsigned long)sg >> RADIX_TREE_EXCEPTIONAL_SHIFT;
+
+ sg = radix_tree_lookup(&iter->radix, base);
+ GEM_BUG_ON(!sg);
+
+ *offset = n - base;
+ }
+
+ rcu_read_unlock();
+
+ return sg;
+}
+
+struct page *
+i915_gem_object_get_page(struct drm_i915_gem_object *obj, unsigned long n)
+{
+ struct scatterlist *sg;
+ unsigned int offset;
+
+ GEM_BUG_ON(!i915_gem_object_has_struct_page(obj));
+
+ sg = i915_gem_object_get_sg(obj, n, &offset);
+ return nth_page(sg_page(sg), offset);
+}
+
+/* Like i915_gem_object_get_page(), but mark the returned page dirty */
+struct page *
+i915_gem_object_get_dirty_page(struct drm_i915_gem_object *obj,
+ unsigned long n)
+{
+ struct page *page;
+
+ page = i915_gem_object_get_page(obj, n);
+ if (!obj->dirty)
+ set_page_dirty(page);
+
+ return page;
+}
+
+dma_addr_t
+i915_gem_object_get_dma_address(struct drm_i915_gem_object *obj,
+ unsigned long n)
+{
+ struct scatterlist *sg;
+ unsigned int offset;
+
+ sg = i915_gem_object_get_sg(obj, n, &offset);
+ return sg_dma_address(sg) + (offset << PAGE_SHIFT);
+}
diff --git a/drivers/gpu/drm/i915/i915_gem_stolen.c b/drivers/gpu/drm/i915/i915_gem_stolen.c
index f4f6d3a48b05..70e61bc35c60 100644
--- a/drivers/gpu/drm/i915/i915_gem_stolen.c
+++ b/drivers/gpu/drm/i915/i915_gem_stolen.c
@@ -595,8 +595,8 @@ _i915_gem_object_create_stolen(struct drm_device *dev,
if (obj->pages == NULL)
goto cleanup;
- obj->get_page.sg = obj->pages->sgl;
- obj->get_page.last = 0;
+ obj->get_page.sg_pos = obj->pages->sgl;
+ obj->get_page.sg_idx = 0;
i915_gem_object_pin_pages(obj);
obj->stolen = stolen;
diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 1c891b92ac80..cb95789da76e 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -526,8 +526,8 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
if (ret == 0) {
list_add_tail(&obj->global_list,
&to_i915(dev)->mm.unbound_list);
- obj->get_page.sg = obj->pages->sgl;
- obj->get_page.last = 0;
+ obj->get_page.sg_pos = obj->pages->sgl;
+ obj->get_page.sg_idx = 0;
Almost like these ones would be better in a helper like
i915_gem_object_init_page_lookup or something.
pinned = 0;
}
}
Regards,
Tvrtko
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