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

[Peng Zhang]

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;