Re: [PATCH v2 14/16] maple_tree: Refine mas_preallocate() node calculations

From: Peng Zhang
Date: Sat Jun 24 2023 - 23:34:37 EST




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;