Re: [RFC PATCH v2] bpf: Using binary search to improve the performance of btf_find_by_name_kind

[Eduard Zingerman]

On Wed, 2023-09-13 at 14:34 +0100, 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..

Hi Alan,

libbpf might be the right place to do this, however, I think that it is
also doable in pahole's btf_encoder__encode(), e.g. as follows:
- after a call to btf__dedup():
  - create a sorted by name IDs ordering;
  - create a new BTF object;
  - add records to the new BTF according to the sorted ordering;
  - remap id references while adding;
  - use the new BTF object instead of old one to write BTF output.

I assume that implementation would be similar regardless whether it is
done in pahole or in libbpf.

Thanks,
Eduard

> 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.
> 
> 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!
> 
> Alan