Date:   Mon, 26 Jun 2023 16:30:49 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        linux-kernel@...r.kernel.org, David Airlie <airlied@...hat.com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 14/16] maple_tree: Refine mas_preallocate() node
 calculations

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@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@...cle.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;
>>>>
>>>>
>>>
>>
> 

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