在 2023/6/23 00:41, Danilo Krummrich 写道:
On 6/12/23 22:39, Liam R. Howlett wrote:
Calculate the number of nodes based on the pending write action instead
of assuming the worst case.
Liam already gave me a heads-up on this patch, which I already replied
to [1].
However, I think it might make sense to also reply to this patch directly.
For a mas_preallocate() calculating the actual required nodes to be
allocated instead of assuming the worst to work, it is required to
ensure that the tree does not change between calling mas_preallocate()
and mas_store_prealloc() if my understanding is correct.
In DRM however, more specifically the DRM GPUVA Manager [2], we do have
the case that we are not able to ensure this:
Jobs to create GPU mappings can be submitted by userspace, are queued up
by the kernel and are processed asynchronously in dma-fence signalling
critical paths, e.g. by using the drm_gpu_scheduler. Hence, we must be
able to allocate the worst case amount of node, since at the time a job
is submitted we can't predict the state the maple tree keeping track of
mappings has once a mapping is inserted in the (asynchronous) dma-fence
signalling critical path.
A more detailed explanation can be found in [1].
Could we keep a separate function for allocating the worst case amount
of nodes additionally to this optimization? E.g. something like
mas_preallocate_worst_case() or mas_preallocate_unlocked() (since I
guess the new one requires the maple tree to be kept locked in order not
to change)?
Hi Danilo,
Your understanding seems incorrect. Even with previously unoptimized
mas_preallocate(), the maple tree cannot be modified between calls to
mas_preallocate() and mas_store_prealloc(). The calculation of the
number of pre-allocated nodes depends on the structure of the maple
tree. In the unoptimized mas_preallocate(), it depends on the height of
the tree. If the maple tree is modified before mas_store_prealloc() and
the height of the tree changes, the number of pre-allocated nodes is
inaccurate.
Regards,
Peng
[1]
https://lore.kernel.org/nouveau/68cd25de-e767-725e-2e7b-703217230bb0@xxxxxxxxxx/T/#ma326e200b1de1e3c9df4e9fcb3bf243061fee8b5
[2]
https://lore.kernel.org/linux-mm/20230620004217.4700-8-dakr@xxxxxxxxxx/T/#m47ab82310f87793d0f0cc1825a316eb30ad5b653
- Danilo
This addresses a performance regression introduced in platforms that
have longer allocation timing.
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
lib/maple_tree.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 47 insertions(+), 1 deletion(-)
diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 048d6413a114..7ac5b5457603 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5541,9 +5541,55 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc);
*/
int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
{
+ MA_WR_STATE(wr_mas, mas, entry);
+ unsigned char node_size;
+ int request = 1;
int ret;
- mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp);
+
+ if (unlikely(!mas->index && mas->last == ULONG_MAX))
+ goto ask_now;
+
+ mas_wr_store_setup(&wr_mas);
+ wr_mas.content = mas_start(mas);
+ /* Root expand */
+ if (unlikely(mas_is_none(mas) || mas_is_ptr(mas)))
+ goto ask_now;
+
+ if (unlikely(!mas_wr_walk(&wr_mas))) {
+ /* Spanning store, use worst case for now */
+ request = 1 + mas_mt_height(mas) * 3;
+ goto ask_now;
+ }
+
+ /* At this point, we are at the leaf node that needs to be
altered. */
+ /* Exact fit, no nodes needed. */
+ if (wr_mas.r_min == mas->index && wr_mas.r_max == mas->last)
+ return 0;
+
+ mas_wr_end_piv(&wr_mas);
+ node_size = mas_wr_new_end(&wr_mas);
+ /* Slot store can avoid using any nodes */
+ if (node_size == wr_mas.node_end && wr_mas.offset_end -
mas->offset == 1)
+ return 0;
+
+ if (node_size >= mt_slots[wr_mas.type]) {
+ /* Split, worst case for now. */
+ request = 1 + mas_mt_height(mas) * 2;
+ goto ask_now;
+ }
+
+ /* Appending does not need any nodes */
+ if (node_size == wr_mas.node_end + 1 && mas->offset ==
wr_mas.node_end)
+ return 0;
+
+ /* Potential spanning rebalance collapsing a node, use worst-case */
+ if (node_size - 1 <= mt_min_slots[wr_mas.type])
+ request = mas_mt_height(mas) * 2 - 1;
+
+ /* node store needs one node */
+ask_now:
+ mas_node_count_gfp(mas, request, gfp);
mas->mas_flags |= MA_STATE_PREALLOC;
if (likely(!mas_is_err(mas)))
return 0;
