Re: [PATCH 06/11] maple_tree: Introduce mas_replace_entry() to directly replace an entry

[Peng Zhang <zhangpeng.00@bytedance.com>]

在 2023/8/1 00:48, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@bytedance.com> [230731 08:39]:
>>
>>
>> 在 2023/7/27 00:08, Liam R. Howlett 写道:
>>> * Peng Zhang <zhangpeng.00@bytedance.com> [230726 04:10]:
>>>> If mas has located a specific entry, it may be need to replace this
>>>> entry, so introduce mas_replace_entry() to do this. mas_replace_entry()
>>>> will be more efficient than mas_store*() because it doesn't do many
>>>> unnecessary checks.
>>>>
>>>> This function should be inline, but more functions need to be moved to
>>>> the header file, so I didn't do it for the time being.
>>>
>>> I am really nervous having no checks here.  I get that this could be
>>> used for duplicating the tree more efficiently, but having a function
>>> that just swaps a value in is very dangerous - especially since it is
>>> decoupled from the tree duplication code.
>> I've thought about this, and I feel like this is something the user
>> should be guaranteed. If the user is not sure whether to use it,
>> mas_store() can be used instead.
> 
> Documentation often isn't up to date and even more rarely read.
> mas_replace_entry() does not give a hint of a requirement for a specific
> state to the mas.  This is not acceptable.
> 
> The description of the function also doesn't say anything about a
> requirement of the maple state, just that it replaces an already
> existing entry.  You have to read the notes to find out that 'mas must
> already locate an existing entry'.
> 
>> And we should provide this interface
>> because it has better performance.
> 
> How much better is the performance?  There's always a trade off but
> without numbers, this is hard to justify.
I have implemented a new version of this pachset, and I will post it
soon.

I tested the benefits of mas_replace_entry() in userspace.
The test code is attached at the end.

Run three times:
mas_replace_entry(): 2.7613050s 2.7120030s 2.7274200s
mas_store():         3.8451260s 3.8113200s 3.9334160s

Using mas_store() reduces the performance of duplicating VMAs by about
41%.

So I think mas_replace_entry() is necessary. We can describe it in more
detail in the documentation to prevent users from misusing it.


static noinline void __init bench_forking(struct maple_tree *mt)
{
	struct maple_tree newmt;
	int i, nr_entries = 134, nr_fork = 80000, ret;
	void *val;
	MA_STATE(mas, mt, 0, 0);
	MA_STATE(newmas, &newmt, 0, 0);
	clock_t start;
	clock_t end;
	double cpu_time_used = 0;

	for (i = 0; i <= nr_entries; i++)
		mtree_store_range(mt, i*10, i*10 + 5,
				  xa_mk_value(i), GFP_KERNEL);

	for (i = 0; i < nr_fork; i++) {
		mt_set_non_kernel(99999);

		start = clock();
		mt_init_flags(&newmt, MT_FLAGS_ALLOC_RANGE);
		mas_lock(&newmas);
		mas_lock(&mas);
		ret = __mt_dup(mt, &newmt, GFP_NOWAIT | __GFP_NOWARN);
		if (ret) {
			pr_err("OOM!");
			BUG_ON(1);
		}

		mas_set(&newmas, 0);
		mas_for_each(&newmas, val, ULONG_MAX) {
			mas_replace_entry(&newmas, val);
		}

		mas_unlock(&mas);
		mas_unlock(&newmas);
		end = clock();
		cpu_time_used += ((double) (end - start));

		mas_destroy(&newmas);
		mt_validate(&newmt);
		mt_set_non_kernel(0);
		mtree_destroy(&newmt);
	}
	printf("time consumption:%.7fs\n", cpu_time_used / CLOCKS_PER_SEC);
}


> 
>>>
>>>>
>>>> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
>>>> ---
>>>>    include/linux/maple_tree.h |  1 +
>>>>    lib/maple_tree.c           | 25 +++++++++++++++++++++++++
>>>>    2 files changed, 26 insertions(+)
>>>>
>>>> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
>>>> index 229fe78e4c89..a05e9827d761 100644
>>>> --- a/include/linux/maple_tree.h
>>>> +++ b/include/linux/maple_tree.h
>>>> @@ -462,6 +462,7 @@ struct ma_wr_state {
>>>>    void *mas_walk(struct ma_state *mas);
>>>>    void *mas_store(struct ma_state *mas, void *entry);
>>>> +void mas_replace_entry(struct ma_state *mas, void *entry);
>>>>    void *mas_erase(struct ma_state *mas);
>>>>    int mas_store_gfp(struct ma_state *mas, void *entry, gfp_t gfp);
>>>>    void mas_store_prealloc(struct ma_state *mas, void *entry);
>>>> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>>>> index efac6761ae37..d58572666a00 100644
>>>> --- a/lib/maple_tree.c
>>>> +++ b/lib/maple_tree.c
>>>> @@ -5600,6 +5600,31 @@ void *mas_store(struct ma_state *mas, void *entry)
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(mas_store);
>>>> +/**
>>>> + * mas_replace_entry() - Replace an entry that already exists in the maple tree
>>>> + * @mas: The maple state
>>>> + * @entry: The entry to store
>>>> + *
>>>> + * Please note that mas must already locate an existing entry, and the new entry
>>>> + * must not be NULL. If these two points cannot be guaranteed, please use
>>>> + * mas_store*() instead, otherwise it will cause an internal error in the maple
>>>> + * tree. This function does not need to allocate memory, so it must succeed.
>>>> + */
>>>> +void mas_replace_entry(struct ma_state *mas, void *entry)
>>>> +{
>>>> +	void __rcu **slots;
>>>> +
>>>> +#ifdef CONFIG_DEBUG_MAPLE_TREE
>>>> +	MAS_WARN_ON(mas, !mte_is_leaf(mas->node));
>>>> +	MAS_WARN_ON(mas, !entry);
>>>> +	MAS_WARN_ON(mas, mas->offset >= mt_slots[mte_node_type(mas->node)]);
>>>> +#endif
>>>> +
>>>> +	slots = ma_slots(mte_to_node(mas->node), mte_node_type(mas->node));
>>>> +	rcu_assign_pointer(slots[mas->offset], entry);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(mas_replace_entry);
>>>> +
>>>>    /**
>>>>     * mas_store_gfp() - Store a value into the tree.
>>>>     * @mas: The maple state
>>>> -- 
>>>> 2.20.1
>>>>