Re: [RFC PATCH v2] bpf: Using binary search to improve the performance of btf_find_by_name_kind
From: pengdonglin
Date: Thu Sep 14 2023 - 06:23:15 EST
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@xxxxxxxxx>
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@xxxxxxxxx>
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@xxxxxxxxxxxxxx/
We can obtain the return values of almost every function during the execution
of kernel through this feature, it can help us analyze problems.
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.
Alan