perf lock contention: Load kernel map before lookup

[PATCH] perf lock contention: Load kernel map before lookup

Posted by Namhyung Kim 1 month, 2 weeks ago

On some machines, it caused troubles when it tried to find kernel
symbols.  I think it's because kernel modules and kallsyms are messed
up during load and split.

Basically we want to make sure the kernel map is loaded and the code has
it in the lock_contention_read().  But recently we added more lookups in
the lock_contention_prepare() which is called before _read().

Also the kernel map (kallsyms) may not be the first one in the group
like on ARM.  Let's use machine__kernel_map() rather than just loading
the first map.

Fixes: 688d2e8de231c54e ("perf lock contention: Add -l/--lock-addr option")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_lock_contention.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 60b81d586323f379..7b5671f13c53525d 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -184,6 +184,9 @@ int lock_contention_prepare(struct lock_contention *con)
 	struct evlist *evlist = con->evlist;
 	struct target *target = con->target;
 
+	/* make sure it loads the kernel map before lookup */
+	map__load(machine__kernel_map(con->machine));
+
 	skel = lock_contention_bpf__open();
 	if (!skel) {
 		pr_err("Failed to open lock-contention BPF skeleton\n");
@@ -749,9 +752,6 @@ int lock_contention_read(struct lock_contention *con)
 		bpf_prog_test_run_opts(prog_fd, &opts);
 	}
 
-	/* make sure it loads the kernel map */
-	maps__load_first(machine->kmaps);
-
 	prev_key = NULL;
 	while (!bpf_map_get_next_key(fd, prev_key, &key)) {
 		s64 ls_key;
-- 
2.51.1.851.g4ebd6896fd-goog

Re: [PATCH] perf lock contention: Load kernel map before lookup

Posted by Namhyung Kim 1 month, 1 week ago

On Wed, 29 Oct 2025 21:01:39 -0700, Namhyung Kim wrote:
> On some machines, it caused troubles when it tried to find kernel
> symbols.  I think it's because kernel modules and kallsyms are messed
> up during load and split.
> 
> Basically we want to make sure the kernel map is loaded and the code has
> it in the lock_contention_read().  But recently we added more lookups in
> the lock_contention_prepare() which is called before _read().
> 
> [...]
Applied to perf-tools-next, thanks!

Best regards,
Namhyung

Re: [PATCH] perf lock contention: Load kernel map before lookup

Posted by Ian Rogers 1 month, 1 week ago

On Wed, Oct 29, 2025 at 9:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On some machines, it caused troubles when it tried to find kernel
> symbols.  I think it's because kernel modules and kallsyms are messed
> up during load and split.
>
> Basically we want to make sure the kernel map is loaded and the code has
> it in the lock_contention_read().  But recently we added more lookups in
> the lock_contention_prepare() which is called before _read().
>
> Also the kernel map (kallsyms) may not be the first one in the group
> like on ARM.  Let's use machine__kernel_map() rather than just loading
> the first map.
>
> Fixes: 688d2e8de231c54e ("perf lock contention: Add -l/--lock-addr option")
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Reviewed-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  tools/perf/util/bpf_lock_contention.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> index 60b81d586323f379..7b5671f13c53525d 100644
> --- a/tools/perf/util/bpf_lock_contention.c
> +++ b/tools/perf/util/bpf_lock_contention.c
> @@ -184,6 +184,9 @@ int lock_contention_prepare(struct lock_contention *con)
>         struct evlist *evlist = con->evlist;
>         struct target *target = con->target;
>
> +       /* make sure it loads the kernel map before lookup */
> +       map__load(machine__kernel_map(con->machine));
> +
>         skel = lock_contention_bpf__open();
>         if (!skel) {
>                 pr_err("Failed to open lock-contention BPF skeleton\n");
> @@ -749,9 +752,6 @@ int lock_contention_read(struct lock_contention *con)
>                 bpf_prog_test_run_opts(prog_fd, &opts);
>         }
>
> -       /* make sure it loads the kernel map */
> -       maps__load_first(machine->kmaps);
> -
>         prev_key = NULL;
>         while (!bpf_map_get_next_key(fd, prev_key, &key)) {
>                 s64 ls_key;
> --
> 2.51.1.851.g4ebd6896fd-goog
>