On Mon, 23 Feb 2026 16:51:12 -0500 Andrey Grodzovsky <[email protected]> wrote:
> When ftrace_lookup_symbols() is called with a single symbol (cnt == 1), > use kallsyms_lookup_name() for O(log N) binary search instead of the > full linear scan via kallsyms_on_each_symbol(). So this patch looks like it should go through the tracing tree, not bpf. > > ftrace_lookup_symbols() was designed for batch resolution of many > symbols in a single pass. For large cnt this is efficient: a single > O(N) walk over all symbols with O(log cnt) binary search into the > sorted input array. But for cnt == 1 it still decompresses all ~200K > kernel symbols only to match one. > > kallsyms_lookup_name() uses the sorted kallsyms index and needs only > ~17 decompressions for a single lookup. > > This is the common path for kprobe.session with exact function names, > where libbpf sends one symbol per BPF_LINK_CREATE syscall. > > If binary lookup fails (duplicate symbol names where the first match > is not ftrace-instrumented, or module symbols), the function falls > through to the existing linear scan path. > > Before (cnt=1, 50 kprobe.session programs): > Attach: 858 ms (kallsyms_expand_symbol 25% of CPU) > > After: > Attach: 52 ms (16x faster) > > Signed-off-by: Andrey Grodzovsky <[email protected]> > --- > kernel/trace/ftrace.c | 28 ++++++++++++++++++++++++++++ > 1 file changed, 28 insertions(+) > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index 827fb9a0bf0d..bfd7670669c2 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -9263,6 +9263,19 @@ static int kallsyms_callback(void *data, const char > *name, unsigned long addr) > * @addrs array, which needs to be big enough to store at least @cnt > * addresses. > * > + * For a single symbol (cnt == 1), uses kallsyms_lookup_name() which > + * performs an O(log N) binary search via the sorted kallsyms index. > + * This avoids the full O(N) linear scan over all kernel symbols that > + * the multi-symbol path requires. > + * > + * For multiple symbols, uses a single-pass linear scan via > + * kallsyms_on_each_symbol() with binary search into the sorted input > + * array. The above is fine. > While individual lookups are O(log N), doing K lookups > + * totals O(K * log N) which loses to a single sequential O(N) pass > + * at scale due to cache-friendly memory access patterns of the linear > + * walk. Empirical testing shows the linear scan is faster for batch > + * lookups even well below 10K symbols. The above is unneeded for a comment in the code and just belongs in the change log. -- Steve > + * > * Returns: 0 if all provided symbols are found, -ESRCH otherwise. > */ > int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned > long *addrs) > @@ -9270,6 +9283,21 @@ int ftrace_lookup_symbols(const char **sorted_syms, > size_t cnt, unsigned long *a > struct kallsyms_data args; > int found_all; > > + /* Fast path: single symbol uses O(log N) binary search */ > + if (cnt == 1) { > + addrs[0] = kallsyms_lookup_name(sorted_syms[0]); > + if (addrs[0]) > + addrs[0] = ftrace_location(addrs[0]); > + if (addrs[0]) > + return 0; > + /* > + * Binary lookup can fail for duplicate symbol names > + * where the first match is not ftrace-instrumented, > + * or for module symbols. Retry with linear scan. > + */ > + } > + > + /* Batch path: single-pass O(N) linear scan */ > memset(addrs, 0, sizeof(*addrs) * cnt); > args.addrs = addrs; > args.syms = sorted_syms;
