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

[Danilo Krummrich]

On 6/26/23 16:08, Peng Zhang wrote:
> 在 2023/6/26 08:38, Danilo Krummrich 写道:
> > Hi Peng,
> >
> > On 6/25/23 05:28, Peng Zhang wrote:
> > > 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.
> >
> > Thanks for pointing this out!
> >
> > First of all, it's probably fair to say "naive me", it totally makes 
> > sense the tree height is needed - it's a b-tree.
> >
> > On the other hand, unless I miss something (and if so, please let me 
> > know), something is bogus with the API then.
> >
> > While the documentation of the Advanced API of the maple tree 
> > explicitly claims that the user of the API is responsible for locking, 
> > this should be limited to the bounds set by the maple tree 
> > implementation. Which means, the user must decide for either the 
> > internal (spin-) lock or an external lock (which possibly goes away in 
> > the future) and acquire and release it according to the rules maple 
> > tree enforces through lockdep checks.
> >
> > Let's say one picks the internal lock. How is one supposed to ensure 
> > the tree isn't modified using the internal lock with mas_preallocate()?
> >
> > Besides that, I think the documentation should definitely mention this 
> > limitation and give some guidance for the locking.
> Yes, the documentation of maple tree is not detailed and complete.
> > Currently, from an API perspective, I can't see how anyone not 
> > familiar with the implementation details would be able to recognize 
> > this limitation.
> >
> > In terms of the GPUVA manager, unfortunately, it seems like I need to 
> > drop the maple tree and go back to using a rb-tree, since it seems 
> > there is no sane way doing a worst-case pre-allocation that does not 
> > suffer from this limitation.
> I also think preallocation may not be necessary, and I agree with what
> Matthew said. Preallocation should be used in some cases where
> preallocation has to be used. If preallocation is used, but the number
> of preallocated nodes is insufficient because the tree is modified
> midway, GFP_NOWAIT will be used for memory allocation during the tree
> modification process, and the user may not notice that more nodes are
> not from preallocation.

Please see my reply to Matthew. :)

- Danilo

> > - Danilo
> >
> > > 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;