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

[Danilo Krummrich <dakr@redhat.com>]

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)?

[1] 
https://lore.kernel.org/nouveau/68cd25de-e767-725e-2e7b-703217230bb0@redhat.com/T/#ma326e200b1de1e3c9df4e9fcb3bf243061fee8b5

[2] 
https://lore.kernel.org/linux-mm/20230620004217.4700-8-dakr@redhat.com/T/#m47ab82310f87793d0f0cc1825a316eb30ad5b653

- Danilo

> 
> This addresses a performance regression introduced in platforms that
> have longer allocation timing.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>   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;