Re: [PATCH] perf maps: Remove check_invariants() from maps__lock()

[Ian Rogers <irogers@google.com>]

On Mon, Apr 29, 2024 at 3:57 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> I found that the debug build was a lot slowed down by the maps lock code
> since it checks the invariants whenever it gets the pointer to the lock.
> This means it checks twice the invariants before and after the access.
>
> Instead, let's move the checking code within the lock area but after any
> modification and remove it from the read paths.  This would remove (more
> than) the half of the maps lock overhead.
>
> The time for perf report with a huge data file (200k+ of MMAP2 events).
>
>   Non-debug     Before      After
>   ---------   --------   --------
>      2m 43s     6m 45s     4m 21s

Thanks Namhyung, I think the change makes sense. There is an issue
that a user of a map may mutate it in a way that breaks the maps
sorting, symbol code would be a likely culprit of such a thing. A fix
for such a breakage would be to just set the unsorted flag on maps.
We'd be less likely to spot such an issue after this change, but it is
simple enough to add the function call when needed. Another option
would be to add another make flag like PARSER_DEBUG for these kind of,
"be paranoid," type things. Anyway, let's try to fix those problems
when they exist.

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

Thanks,
Ian

> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/maps.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/util/maps.c b/tools/perf/util/maps.c
> index 725300896f38..61eb742d91e3 100644
> --- a/tools/perf/util/maps.c
> +++ b/tools/perf/util/maps.c
> @@ -211,11 +211,6 @@ void maps__set_unwind_libunwind_ops(struct maps *maps, const struct unwind_libun
>
>  static struct rw_semaphore *maps__lock(struct maps *maps)
>  {
> -       /*
> -        * When the lock is acquired or released the maps invariants should
> -        * hold.
> -        */
> -       check_invariants(maps);
>         return &RC_CHK_ACCESS(maps)->lock;
>  }
>
> @@ -358,6 +353,7 @@ static int map__strcmp(const void *a, const void *b)
>  static int maps__sort_by_name(struct maps *maps)
>  {
>         int err = 0;
> +
>         down_write(maps__lock(maps));
>         if (!maps__maps_by_name_sorted(maps)) {
>                 struct map **maps_by_name = maps__maps_by_name(maps);
> @@ -384,6 +380,7 @@ static int maps__sort_by_name(struct maps *maps)
>                         maps__set_maps_by_name_sorted(maps, true);
>                 }
>         }
> +       check_invariants(maps);
>         up_write(maps__lock(maps));
>         return err;
>  }
> @@ -502,6 +499,7 @@ int maps__insert(struct maps *maps, struct map *map)
>
>         down_write(maps__lock(maps));
>         ret = __maps__insert(maps, map);
> +       check_invariants(maps);
>         up_write(maps__lock(maps));
>         return ret;
>  }
> @@ -536,6 +534,7 @@ void maps__remove(struct maps *maps, struct map *map)
>  {
>         down_write(maps__lock(maps));
>         __maps__remove(maps, map);
> +       check_invariants(maps);
>         up_write(maps__lock(maps));
>  }
>
> @@ -602,6 +601,7 @@ void maps__remove_maps(struct maps *maps, bool (*cb)(struct map *map, void *data
>                 else
>                         i++;
>         }
> +       check_invariants(maps);
>         up_write(maps__lock(maps));
>  }
>
> @@ -942,6 +942,8 @@ int maps__copy_from(struct maps *dest, struct maps *parent)
>                         map__put(new);
>                 }
>         }
> +       check_invariants(dest);
> +
>         up_read(maps__lock(parent));
>         up_write(maps__lock(dest));
>         return err;
> @@ -1097,6 +1099,7 @@ void maps__fixup_end(struct maps *maps)
>                 map__set_end(maps_by_address[n - 1], ~0ULL);
>
>         RC_CHK_ACCESS(maps)->ends_broken = false;
> +       check_invariants(maps);
>
>         up_write(maps__lock(maps));
>  }
> @@ -1147,6 +1150,8 @@ int maps__merge_in(struct maps *kmaps, struct map *new_map)
>             map__start(kmaps_maps_by_address[first_after_]) >= map__end(new_map)) {
>                 /* No overlap so regular insert suffices. */
>                 int ret = __maps__insert(kmaps, new_map);
> +
> +               check_invariants(kmaps);
>                 up_write(maps__lock(kmaps));
>                 return ret;
>         }
> @@ -1184,6 +1189,7 @@ int maps__merge_in(struct maps *kmaps, struct map *new_map)
>                 map__zput(kmaps_maps_by_address[i]);
>
>         free(kmaps_maps_by_address);
> +       check_invariants(kmaps);
>         up_write(maps__lock(kmaps));
>         return 0;
>  }
> --
> 2.44.0.769.g3c40516874-goog
>