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ftrace: Use kallsyms binary search for single-symbol lookup

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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().

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), 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)

Cc: <bpf@vger.kernel.org>
Link: https://patch.msgid.link/20260302200837.317907-3-andrey.grodzovsky@crowdstrike.com
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Andrey Grodzovsky
2026-03-02 15:08:36 -05:00
committed by Steven Rostedt (Google)
parent c369299895
commit 93e8fd1a56
1 changed files with 22 additions and 0 deletions
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22
kernel/trace/ftrace.c
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@@ -9267,6 +9267,15 @@ 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.
*
* Returns: 0 if all provided symbols are found, -ESRCH otherwise.
*/
int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs)
@@ -9274,6 +9283,19 @@ 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] && ftrace_location(addrs[0]))
return 0;
/*
* Binary lookup can fail for duplicate symbol names
* where the first match is not ftrace-instrumented.
* 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;