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Message-ID: <CAErzpmvpmx=WM7kHLC-WFbCx0=OpK5f8KJJuOA8gyb7LmRjk2g@mail.gmail.com>
Date: Sat, 20 Dec 2025 17:38:27 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: ast@...nel.org, eddyz87@...il.com, zhangxiaoqin@...omi.com,
ihor.solodrai@...ux.dev, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
pengdonglin <pengdonglin@...omi.com>, Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH bpf-next v10 04/13] libbpf: Optimize type lookup with
binary search for sorted BTF
On Sat, Dec 20, 2025 at 1:28 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Dec 18, 2025 at 6:53 PM Donglin Peng <dolinux.peng@...il.com> wrote:
> >
> > On Fri, Dec 19, 2025 at 7:29 AM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Thu, Dec 18, 2025 at 3:31 AM Donglin Peng <dolinux.peng@...il.com> wrote:
> > > >
> > > > From: pengdonglin <pengdonglin@...omi.com>
> > > >
> > > > This patch introduces binary search optimization for BTF type lookups
> > > > when the BTF instance contains sorted types.
> > > >
> > > > The optimization significantly improves performance when searching for
> > > > types in large BTF instances with sorted types. For unsorted BTF, the
> > > > implementation falls back to the original linear search.
> > > >
> > > > Cc: Eduard Zingerman <eddyz87@...il.com>
> > > > Cc: Alexei Starovoitov <ast@...nel.org>
> > > > Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > > > Cc: Alan Maguire <alan.maguire@...cle.com>
> > > > Cc: Ihor Solodrai <ihor.solodrai@...ux.dev>
> > > > Cc: Xiaoqin Zhang <zhangxiaoqin@...omi.com>
> > > > Signed-off-by: pengdonglin <pengdonglin@...omi.com>
> > > > ---
> > > > tools/lib/bpf/btf.c | 103 ++++++++++++++++++++++++++++++++++----------
> > > > 1 file changed, 80 insertions(+), 23 deletions(-)
> > > >
> > >
> > > [...]
> > >
> > > > + l = start_id;
> > > > + r = end_id;
> > > > + while (l <= r) {
> > > > + m = l + (r - l) / 2;
> > > > + t = btf_type_by_id(btf, m);
> > > > + tname = btf__str_by_offset(btf, t->name_off);
> > > > + ret = strcmp(tname, name);
> > > > + if (ret < 0) {
> > > > + l = m + 1;
> > > > + } else {
> > > > + if (ret == 0)
> > > > + lmost = m;
> > > > + r = m - 1;
> > > > + }
> > > > }
> > >
> > > this differs from what we discussed in [0], you said you'll use that
> > > approach. Can you please elaborate on why you didn't?
> > >
> > > [0] https://lore.kernel.org/bpf/CAEf4Bzb3Eu0J83O=Y4KA-LkzBMjtx7cbonxPzkiduzZ1Pedajg@mail.gmail.com/
> >
> > Yes. As mentioned in the v8 changelog [1], the binary search approach
> > you referenced was implemented in versions v6 and v7 [2]. However,
> > testing revealed a slight performance regression. The root cause was
> > an extra strcmp operation introduced in v7, as discussed in [3]. Therefore,
> > in v8, I reverted to the approach from v5 [4] and refactored it for clarity.
>
> If you keep oscillating like that this patch set will never land. 4%
> (500us) gain on artificial and unrealistic micro-benchmark is
> meaningless and irrelevant, you are just adding more work for yourself
> and for reviewers by constantly changing your implementation between
> revisions for no good reason.
Thank you, I understand and will learn from it. I think the performance gain
makes sense. I’d like to share a specific real-world case where this
optimization
could matter: the `btf_find_by_name_kind()` function is indeed infrequently
used by the BPF subsystem, but it’s heavily relied upon by the ftrace
subsystem’s features like `func-args`, `funcgraph-args` [1], and the upcoming
`funcgraph-retval` [2]. These features invoke the function nearly once per
trace line when outputting, with a call frequency that can reach **100 kHz**
in intensive tracing workloads.
In such scenarios, the extra `strcmp` operations translate to ~100,000
additional
string comparisons per second. While this might seem negligible in isolation,
the overhead accumulates under high-frequency tracing—potentially impacting
latency for users relying on detailed function argument/return value tracing.
Thanks again for pushing for rigor—it helps make the code more cleaner
and robust.
[1] https://lore.kernel.org/all/20250227185822.639418500@goodmis.org/
[2] https://lore.kernel.org/all/20251215034153.2367756-1-dolinux.peng@gmail.com/
>
>
> >
> > Benchmark results show that v8 achieves a 4.2% performance improvement
> > over v7. If we don't care the performance gain, I will revert to the approach
> > in v7 in the next version.
> >
> > [1] https://lore.kernel.org/bpf/20251126085025.784288-1-dolinux.peng@gmail.com/
> > [2] https://lore.kernel.org/all/20251119031531.1817099-1-dolinux.peng@gmail.com/
> > [3] https://lore.kernel.org/all/CAEf4BzaqEPD46LddJHO1-k5KPGyVWf6d=duDAxG1q=jykJkMBg@mail.gmail.com/
> > [4] https://lore.kernel.org/all/20251106131956.1222864-4-dolinux.peng@gmail.com/
> >
> > >
> > > >
> > > > - return libbpf_err(-ENOENT);
> > > > + return lmost;
> > > > }
> > > >
> > > > static __s32 btf_find_by_name_kind(const struct btf *btf, int start_id,
> > > > const char *type_name, __u32 kind)
> > >
> > > kind is defined as u32 but you expect caller to pass -1 to ignore the
> > > kind. Use int here.
> >
> > Thanks, I will fix it.
> >
> > >
> > > > {
> > > > - __u32 i, nr_types = btf__type_cnt(btf);
> > > > + const struct btf_type *t;
> > > > + const char *tname;
> > > > + __s32 idx;
> > > > +
> > > > + if (start_id < btf->start_id) {
> > > > + idx = btf_find_by_name_kind(btf->base_btf, start_id,
> > > > + type_name, kind);
> > > > + if (idx >= 0)
> > > > + return idx;
> > > > + start_id = btf->start_id;
> > > > + }
> > > >
> > > > - if (kind == BTF_KIND_UNKN || !strcmp(type_name, "void"))
> > > > + if (kind == BTF_KIND_UNKN || strcmp(type_name, "void") == 0)
> > > > return 0;
> > > >
> > > > - for (i = start_id; i < nr_types; i++) {
> > > > - const struct btf_type *t = btf__type_by_id(btf, i);
> > > > - const char *name;
> > > > + if (btf->sorted_start_id > 0 && type_name[0]) {
> > > > + __s32 end_id = btf__type_cnt(btf) - 1;
> > > > +
> > > > + /* skip anonymous types */
> > > > + start_id = max(start_id, btf->sorted_start_id);
> > >
> > > can sorted_start_id ever be smaller than start_id?
> > >
> > > > + idx = btf_find_by_name_bsearch(btf, type_name, start_id, end_id);
> > >
> > > is there ever a time when btf_find_by_name_bsearch() will work with
> > > different start_id and end_id? why is this not done inside the
> > > btf_find_by_name_bsearch()?
> >
> > Because the start_id could be specified by the caller.
>
> Right, start_id has to be passed in. But end_id is always the same, so
> maybe determine it internally instead? And let's not return -ENOENT
Thanks, I agree and will put the end_id into btf_find_by_name_bsearch.
> from btf_find_by_name_bsearch(), as I mentioned before, it would be
> more streamlined if you return btf__type_cnt(btf) if search failed.
Thanks, I agree.
>
> >
> > >
> > > > + if (unlikely(idx < 0))
> > > > + return libbpf_err(-ENOENT);
> > >
> > > pass through error returned from btf_find_by_name_bsearch(), why redefining it?
> >
> > Thanks, I will fix it.
> >
>
> see above, by returning btf__type_cnt() you won't even have this error
> handling, you'll just go through normal loop checking for a match and
> won't find anything, returning -ENOENT then.
Thanks, I agree.
>
> > >
> > > > +
> > > > + if (unlikely(kind == -1))
> > > > + return idx;
> > > > +
> > > > + t = btf_type_by_id(btf, idx);
> > > > + if (likely(BTF_INFO_KIND(t->info) == kind))
> > >
> > > use btf_kind(), but this whole extra check is just unnecessary, this
> >
> > Thanks, I will do it.
> >
> > > should be done in the loop below. We talked about all this already,
> > > why do I feel like I'm being ignored?..
> >
> > Sorry for the confusion, and absolutely not ignoring you.
> >
>
> If you decide to change implementation due to some unforeseen factors
> (like concern about 4% microbenchmark improvement), it would be
> helpful for you to call this out in a reply to the original
> discussion. A line somewhere in the cover letter changelog is way too
> easy to miss and that doesn't give me an opportunity to stop you
> before you go and produce another revision that I'll then be
> rejecting.
I will learn from it and thank you for the suggestion.
>
> > >
> > > > + return idx;
> > >
> > > drop all these likely and unlikely micro optimizations, please
> >
> > Thanks, I will do it.
> >
> > >
> > >
> > > > +
> > > > + for (idx++; idx <= end_id; idx++) {
> > > > + t = btf__type_by_id(btf, idx);
> > > > + tname = btf__str_by_offset(btf, t->name_off);
> > > > + if (strcmp(tname, type_name) != 0)
> > > > + return libbpf_err(-ENOENT);
> > > > + if (btf_kind(t) == kind)
> > > > + return idx;
> > > > + }
> > > > + } else {
> > > > + __u32 i, total;
> > > >
> > > > - if (btf_kind(t) != kind)
> > > > - continue;
> > > > - name = btf__name_by_offset(btf, t->name_off);
> > > > - if (name && !strcmp(type_name, name))
> > > > - return i;
> > > > + total = btf__type_cnt(btf);
> > > > + for (i = start_id; i < total; i++) {
> > > > + t = btf_type_by_id(btf, i);
> > > > + if (kind != -1 && btf_kind(t) != kind)
> > >
> > > nit: kind < 0, no need to hard-code -1
> >
> > Good, I will fix it.
> >
> > >
> > > > + continue;
> > > > + tname = btf__str_by_offset(btf, t->name_off);
> > > > + if (strcmp(tname, type_name) == 0)
> > > > + return i;
> > > > + }
> > > > }
> > > >
> > > > return libbpf_err(-ENOENT);
> > > > }
> > > >
> > >
> > > [...]
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