Message-ID: <774732d7-1603-466e-8df2-3b21314913e5@sangfor.com.cn>
Date:   Thu, 14 Sep 2023 21:05:06 +0800
From:   pengdonglin <pengdonglin@...gfor.com.cn>
To:     Alan Maguire <alan.maguire@...cle.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Eduard Zingerman <eddyz87@...il.com>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, dinghui@...gfor.com.cn,
        huangcun@...gfor.com.cn, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] bpf: Using binary search to improve the
 performance of btf_find_by_name_kind

On 2023/9/14 20:46, Alan Maguire wrote:
> On 14/09/2023 11:13, pengdonglin wrote:
>> On 2023/9/13 21:34, Alan Maguire wrote:
>>> On 13/09/2023 11:32, pengdonglin wrote:
>>>> On 2023/9/13 2:46, Alexei Starovoitov wrote:
>>>>> On Tue, Sep 12, 2023 at 10:03 AM Eduard Zingerman <eddyz87@...il.com>
>>>>> wrote:
>>>>>>
>>>>>> On Tue, 2023-09-12 at 09:40 -0700, Alexei Starovoitov wrote:
>>>>>>> On Tue, Sep 12, 2023 at 7:19 AM Eduard Zingerman <eddyz87@...il.com>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>> On Tue, 2023-09-12 at 16:51 +0300, Eduard Zingerman wrote:
>>>>>>>>> On Sat, 2023-09-09 at 02:16 -0700, Donglin Peng wrote:
>>>>>>>>>> Currently, we are only using the linear search method to find the
>>>>>>>>>> type id
>>>>>>>>>> by the name, which has a time complexity of O(n). This change
>>>>>>>>>> involves
>>>>>>>>>> sorting the names of btf types in ascending order and using
>>>>>>>>>> binary search,
>>>>>>>>>> which has a time complexity of O(log(n)). This idea was inspired
>>>>>>>>>> by the
>>>>>>>>>> following patch:
>>>>>>>>>>
>>>>>>>>>> 60443c88f3a8 ("kallsyms: Improve the performance of
>>>>>>>>>> kallsyms_lookup_name()").
>>>>>>>>>>
>>>>>>>>>> At present, this improvement is only for searching in vmlinux's
>>>>>>>>>> and
>>>>>>>>>> module's BTFs, and the kind should only be BTF_KIND_FUNC or
>>>>>>>>>> BTF_KIND_STRUCT.
>>>>>>>>>>
>>>>>>>>>> Another change is the search direction, where we search the BTF
>>>>>>>>>> first and
>>>>>>>>>> then its base, the type id of the first matched btf_type will be
>>>>>>>>>> returned.
>>>>>>>>>>
>>>>>>>>>> Here is a time-consuming result that finding all the type ids of
>>>>>>>>>> 67,819 kernel
>>>>>>>>>> functions in vmlinux's BTF by their names:
>>>>>>>>>>
>>>>>>>>>> Before: 17000 ms
>>>>>>>>>> After:     10 ms
>>>>>>>>>>
>>>>>>>>>> The average lookup performance has improved about 1700x at the
>>>>>>>>>> above scenario.
>>>>>>>>>>
>>>>>>>>>> However, this change will consume more memory, for example,
>>>>>>>>>> 67,819 kernel
>>>>>>>>>> functions will allocate about 530KB memory.
>>>>>>>>>
>>>>>>>>> Hi Donglin,
>>>>>>>>>
>>>>>>>>> I think this is a good improvement. However, I wonder, why did you
>>>>>>>>> choose to have a separate name map for each BTF kind?
>>>>>>>>>
>>>>>>>>> I did some analysis for my local testing kernel config and got
>>>>>>>>> such numbers:
>>>>>>>>> - total number of BTF objects: 97350
>>>>>>>>> - number of FUNC and STRUCT objects: 51597
>>>>>>>>> - number of FUNC, STRUCT, UNION, ENUM, ENUM64, TYPEDEF, DATASEC
>>>>>>>>> objects: 56817
>>>>>>>>>      (these are all kinds for which lookup by name might make sense)
>>>>>>>>> - number of named objects: 54246
>>>>>>>>> - number of name collisions:
>>>>>>>>>      - unique names: 53985 counts
>>>>>>>>>      - 2 objects with the same name: 129 counts
>>>>>>>>>      - 3 objects with the same name: 3 counts
>>>>>>>>>
>>>>>>>>> So, it appears that having a single map for all named objects makes
>>>>>>>>> sense and would also simplify the implementation, what do you
>>>>>>>>> think?
>>>>>>>>
>>>>>>>> Some more numbers for my config:
>>>>>>>> - 13241 types (struct, union, typedef, enum), log2 13241 = 13.7
>>>>>>>> - 43575 funcs, log2 43575 = 15.4
>>>>>>>> Thus, having separate map for types vs functions might save ~1.7
>>>>>>>> search iterations. Is this a significant slowdown in practice?
>>>>>>>
>>>>>>> What do you propose to do in case of duplicates ?
>>>>>>> func and struct can have the same name, but they will have two
>>>>>>> different
>>>>>>> btf_ids. How do we store them ?
>>>>>>> Also we might add global vars to BTF. Such request came up several
>>>>>>> times.
>>>>>>> So we need to make sure our search approach scales to
>>>>>>> func, struct, vars. I don't recall whether we search any other kinds.
>>>>>>> Separate arrays for different kinds seems ok.
>>>>>>> It's a bit of code complexity, but it's not an increase in memory.
>>>>>>
>>>>>> Binary search gives, say, lowest index of a thing with name A, then
>>>>>> increment index while name remains A looking for correct kind.
>>>>>> Given the name conflicts info from above, 99% of times there would be
>>>>>> no need to iterate and in very few cases there would a couple of
>>>>>> iterations.
>>>>>>
>>>>>> Same logic would be necessary with current approach if different BTF
>>>>>> kinds would be allowed in BTF_ID_NAME_* cohorts. I figured that these
>>>>>> cohorts are mainly a way to split the tree for faster lookups, but
>>>>>> maybe that is not the main intent.
>>>>>>
>>>>>>> With 13k structs and 43k funcs it's 56k * (4 + 4) that's 0.5 Mbyte
>>>>>>> extra memory. That's quite a bit. Anything we can do to compress it?
>>>>>>
>>>>>> That's an interesting question, from the top of my head:
>>>>>> pre-sort in pahole (re-assign IDs so that increasing ID also would
>>>>>> mean "increasing" name), shouldn't be that difficult.
>>>>>
>>>>> That sounds great. kallsyms are pre-sorted at build time.
>>>>> We should do the same with BTF.
>>>>> I think GCC can emit BTF directly now and LLVM emits it for bpf progs
>>>>> too,
>>>>> but since vmlinux and kernel module BTFs will keep being processed
>>>>> through pahole we don't have to make gcc/llvm sort things right away.
>>>>> pahole will be enough. The kernel might do 'is it sorted' check
>>>>> during BTF validation and then use binary search or fall back to linear
>>>>> when not-sorted == old pahole.
>>>>>
>>>>
>>>> Yeah, I agree and will attempt to modify the pahole and perform a test.
>>>> Do we need
>>>> to introduce a new macro to control the behavior when the BTF is not
>>>> sorted? If
>>>> it is not sorted, we can use the method mentioned in this patch or use
>>>> linear
>>>> search.
>>>>
>>>>
>>>
>>> One challenge with pahole is that it often runs in parallel mode, so I
>>> suspect any sorting would have to be done after merging across threads.
>>> Perhaps BTF deduplication time might be a useful time to re-sort by
>>> name? BTF dedup happens after BTF has been merged, and a new "sorted"
>>> btf_dedup_opts option could be added and controlled by a pahole
>>> option. However dedup is pretty complicated already..
>>>
>>> One thing we should weigh up though is if there are benefits to the
>>> way BTF is currently laid out. It tends to start with base types,
>>> and often-encountered types end up being located towards the start
>>> of the BTF data. For example
>>>
>>>
>>> [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64
>>> encoding=(none)
>>> [2] CONST '(anon)' type_id=1
>>> [3] VOLATILE '(anon)' type_id=1
>>> [4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
>>> [5] PTR '(anon)' type_id=8
>>> [6] CONST '(anon)' type_id=5
>>> [7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
>>> [8] CONST '(anon)' type_id=7
>>> [9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
>>> [10] CONST '(anon)' type_id=9
>>> [11] TYPEDEF '__s8' type_id=12
>>> [12] INT 'signed char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
>>> [13] TYPEDEF '__u8' type_id=14
>>>
>>> So often-used types will be found quickly, even under linear search
>>> conditions.
>>
>> I found that there seems to be no code in the kernel that get the ID of the
>> basic data type by calling btf_find_by_name_kind directly. The general
>> usage
>> of this function is to obtain the ID of a structure or function. After
>> we got
>> the ID of a structure or function, it is O(1) to get the IDs of its members
>> or parameters.
>>
>> ./kernel/trace/trace_probe.c:383:       id = btf_find_by_name_kind(btf,
>> funcname, BTF_KIND_FUNC);
>> ./kernel/bpf/btf.c:3523:        id = btf_find_by_name_kind(btf,
>> value_type, BTF_KIND_STRUCT);
>> ./kernel/bpf/btf.c:5504:                id = btf_find_by_name_kind(btf,
>> alloc_obj_fields[i], BTF_KIND_STRUCT);
>> ./kernel/bpf/bpf_struct_ops.c:128:      module_id =
>> btf_find_by_name_kind(btf, "module", BTF_KIND_STRUCT);
>> ./net/ipv4/bpf_tcp_ca.c:28:     type_id = btf_find_by_name_kind(btf,
>> "sock", BTF_KIND_STRUCT);
>> ./net/ipv4/bpf_tcp_ca.c:33:     type_id = btf_find_by_name_kind(btf,
>> "tcp_sock", BTF_KIND_STRUCT);
>> ./net/netfilter/nf_bpf_link.c:181:      type_id =
>> btf_find_by_name_kind(btf, name, BTF_KIND_STRUCT);
>>
>>>
>>> When we look at how many lookups by id (which are O(1), since they are
>>> done via the btf->types[] array) versus by name, we see:
>>>
>>> $ grep btf_type_by_id kernel/bpf/*.c|wc -l
>>> 120
>>> $ grep btf_find_by_nam kernel/bpf/*.c|wc -l
>>> 15
>>>
>>> I don't see a huge number of name-based lookups, and I think most are
>>> outside of the hotter codepaths, unless I'm missing some. All of which
>>> is to say it would be a good idea to have a clear sense of what will get
>>> faster with sorted-by-name BTF. Thanks!
>>
>> The story goes like this.
>>
>> I have added a new feature to the function graph called "funcgraph_retval",
>> here is the link:
>>
>> https://lore.kernel.org/all/1fc502712c981e0e6742185ba242992170ac9da8.1680954589.git.pengdonglin@sangfor.com.cn/
>>
>> We can obtain the return values of almost every function during the
>> execution
>> of kernel through this feature, it can help us analyze problems.
>>
> 
> It's a great feature!

Thanks.

> 
>> However, this feature has two main drawbacks.
>>
>> 1. Even if a function's return type is void,  a return value will still
>> be printed.
>>
>> 2. The return value printed may be incorrect when the width of the
>> return type is
>> smaller than the generic register.
>>
>> I think if we can get this return type of the function, then the
>> drawbacks mentioned
>> above can be eliminated. The function btf_find_by_name_kind can be used
>> to get the ID of
>> the kernel function, then we can get its return type easily. If the
>> return type is
>> void, the return value recorded will not be printed. If the width of the
>> return type
>> is smaller than the generic register, then the value stored in the upper
>> bits will be
>> trimmed. I have written a demo and these drawbacks were resolved.
>>
>> However, during my test, I found that it took too much time when read
>> the trace log
>> with this feature enabled, because the trace log consists of 200,000
>> lines. The
>> majority of the time was consumed by the btf_find_by_name_kind, which is
>> called
>> 200,000 times.
>>
>> So I think the performance of btf_find_by_name_kind  may need to be
>> improved.
>>
> 
> If I recall, Masami's work uses BTF ids, but can cache them since the
> user explicitly asks for specific fields in the trace output. I'm
> presuming that's not an option for you due to the fact funcgraph tracing
> enables everything (or at least everything under a filter predicate) and
> you have limited context to work with, is that right?

Yes, right.

> 
> Looking at print_graph_entry_leaf() which I _think_ is where you'd need
> to print the retval from, you have access to the function address via
> call->func, and I presume you get the name from snprinting the symbol to
> a string or similar. So you're stuck in a context where you have the
> function address, and from that you can derive the function name. Is
> that correct? Thanks!

Yes, both print_graph_return and print_graph_entry_leaf will call print_graph_retval
to print the return value. Then call sprint_symbol_no_offset with call->func to get
the function name, then call btf_find_by_name_kind to get the return type.

> 
> Alan
> 

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