Re: [PATCH v3 12/50] perf record: Lazy load kernel symbols

[Namhyung Kim]

Hi Ian,

On Tue, Oct 24, 2023 at 3:24 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> Commit 5b7ba82a7591 ("perf symbols: Load kernel maps before using")
> changed it so that loading a kernel dso would cause the symbols for
> the dso to be eagerly loaded. For perf record this is overhead as the
> symbols won't be used. Add a symbol_conf to control the behavior and
> disable it for perf record and perf inject.

I'm curious if it can simply move to lazy loading unconditionally.
In most cases, the code calls machine__resolve() which calls
thread__find_map() and map__find_symbol() to load symbols.

So I think it's unnecessary to do it in the thread__find_map().
If it needs a symbol, it should call map__find_symbol() first
and it'll load the symbol table.

Adrian, what's special in inject or Intel-PT on this?

Thanks,
Namhyung


>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/builtin-inject.c   | 6 ++++++
>  tools/perf/builtin-record.c   | 2 ++
>  tools/perf/util/event.c       | 4 ++--
>  tools/perf/util/symbol_conf.h | 3 ++-
>  4 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
> index c8cf2fdd9cff..eb3ef5c24b66 100644
> --- a/tools/perf/builtin-inject.c
> +++ b/tools/perf/builtin-inject.c
> @@ -2265,6 +2265,12 @@ int cmd_inject(int argc, const char **argv)
>                 "perf inject [<options>]",
>                 NULL
>         };
> +
> +       if (!inject.itrace_synth_opts.set) {
> +               /* Disable eager loading of kernel symbols that adds overhead to perf inject. */
> +               symbol_conf.lazy_load_kernel_maps = true;
> +       }
> +
>  #ifndef HAVE_JITDUMP
>         set_option_nobuild(options, 'j', "jit", "NO_LIBELF=1", true);
>  #endif
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index dcf288a4fb9a..8ec818568662 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -3989,6 +3989,8 @@ int cmd_record(int argc, const char **argv)
>  # undef set_nobuild
>  #endif
>
> +       /* Disable eager loading of kernel symbols that adds overhead to perf record. */
> +       symbol_conf.lazy_load_kernel_maps = true;
>         rec->opts.affinity = PERF_AFFINITY_SYS;
>
>         rec->evlist = evlist__new();
> diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> index 923c0fb15122..68f45e9e63b6 100644
> --- a/tools/perf/util/event.c
> +++ b/tools/perf/util/event.c
> @@ -617,13 +617,13 @@ struct map *thread__find_map(struct thread *thread, u8 cpumode, u64 addr,
>         if (cpumode == PERF_RECORD_MISC_KERNEL && perf_host) {
>                 al->level = 'k';
>                 maps = machine__kernel_maps(machine);
> -               load_map = true;
> +               load_map = !symbol_conf.lazy_load_kernel_maps;
>         } else if (cpumode == PERF_RECORD_MISC_USER && perf_host) {
>                 al->level = '.';
>         } else if (cpumode == PERF_RECORD_MISC_GUEST_KERNEL && perf_guest) {
>                 al->level = 'g';
>                 maps = machine__kernel_maps(machine);
> -               load_map = true;
> +               load_map = !symbol_conf.lazy_load_kernel_maps;
>         } else if (cpumode == PERF_RECORD_MISC_GUEST_USER && perf_guest) {
>                 al->level = 'u';
>         } else {
> diff --git a/tools/perf/util/symbol_conf.h b/tools/perf/util/symbol_conf.h
> index 0b589570d1d0..2b2fb9e224b0 100644
> --- a/tools/perf/util/symbol_conf.h
> +++ b/tools/perf/util/symbol_conf.h
> @@ -42,7 +42,8 @@ struct symbol_conf {
>                         inline_name,
>                         disable_add2line_warn,
>                         buildid_mmap2,
> -                       guest_code;
> +                       guest_code,
> +                       lazy_load_kernel_maps;
>         const char      *vmlinux_name,
>                         *kallsyms_name,
>                         *source_prefix,
> --
> 2.42.0.758.gaed0368e0e-goog
>