Re: [PATCH v3 1/6] perf maps: Switch from rbtree to lazily sorted array for addresses

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Feb 9, 2024 at 7:18 PM Ian Rogers <irogers@google.com> wrote:
>
> Maps is a collection of maps primarily sorted by the starting address
> of the map. Prior to this change the maps were held in an rbtree
> requiring 4 pointers per node. Prior to reference count checking, the
> rbnode was embedded in the map so 3 pointers per node were
> necessary. This change switches the rbtree to an array lazily sorted
> by address, much as the array sorting nodes by name. 1 pointer is
> needed per node, but to avoid excessive resizing the backing array may
> be twice the number of used elements. Meaning the memory overhead is
> roughly half that of the rbtree. For a perf record with
> "--no-bpf-event -g -a" of true, the memory overhead of perf inject is
> reduce fom 3.3MB to 3MB, so 10% or 300KB is saved.
>
> Map inserts always happen at the end of the array. The code tracks
> whether the insertion violates the sorting property. O(log n) rb-tree
> complexity is switched to O(1).
>
> Remove slides the array, so O(log n) rb-tree complexity is degraded to
> O(n).
>
> A find may need to sort the array using qsort which is O(n*log n), but
> in general the maps should be sorted and so average performance should
> be O(log n) as with the rbtree.
>
> An rbtree node consumes a cache line, but with the array 4 nodes fit
> on a cache line. Iteration is simplified to scanning an array rather
> than pointer chasing.
>
> Overall it is expected the performance after the change should be
> comparable to before, but with half of the memory consumed.
>
> To avoid a list and repeated logic around splitting maps,
> maps__merge_in is rewritten in terms of
> maps__fixup_overlap_and_insert. maps_merge_in splits the given mapping
> inserting remaining gaps. maps__fixup_overlap_and_insert splits the
> existing mappings, then adds the incoming mapping. By adding the new
> mapping first, then re-inserting the existing mappings the splitting
> behavior matches.
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> Acked-by: Namhyung Kim <namhyung@kernel.org>
> ---
[SNIP]
>  int maps__for_each_map(struct maps *maps, int (*cb)(struct map *map, void *data), void *data)
>  {
> -       struct map_rb_node *pos;
> +       bool done = false;
>         int ret = 0;
>
> -       down_read(maps__lock(maps));
> -       maps__for_each_entry(maps, pos) {
> -               ret = cb(pos->map, data);
> -               if (ret)
> -                       break;
> +       /* See locking/sorting note. */
> +       while (!done) {
> +               down_read(maps__lock(maps));
> +               if (maps__maps_by_address_sorted(maps)) {
> +                       /*
> +                        * maps__for_each_map callbacks may buggily/unsafely
> +                        * insert into maps_by_address. Deliberately reload
> +                        * maps__nr_maps and maps_by_address on each iteration
> +                        * to avoid using memory freed by maps__insert growing
> +                        * the array - this may cause maps to be skipped or
> +                        * repeated.
> +                        */
> +                       for (unsigned int i = 0; i < maps__nr_maps(maps); i++) {
> +                               struct map **maps_by_address = maps__maps_by_address(maps);

Any chance they can move out of the loop?  I guess not as they are
not marked to const/pure functions..

Thanks,
Namhyung


> +                               struct map *map = maps_by_address[i];
> +
> +                               ret = cb(map, data);
> +                               if (ret)
> +                                       break;
> +                       }
> +                       done = true;
> +               }
> +               up_read(maps__lock(maps));
> +               if (!done)
> +                       maps__sort_by_address(maps);
>         }
> -       up_read(maps__lock(maps));
>         return ret;
>  }