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

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Message-ID: <CAP-5=fV3H234wnVkmf04kyJYb=M9gUtZbHigrc7pQKYUXoFSzg@mail.gmail.com>
Date: Fri, 31 Oct 2025 09:56:17 -0700
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, James Clark <james.clark@...aro.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf lock contention: Load kernel map before lookup

On Wed, Oct 29, 2025 at 9:01 PM Namhyung Kim <namhyung@...nel.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@...nel.org>

Reviewed-by: Ian Rogers <irogers@...gle.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
>