Re: [PATCH 2/9] lib/ref_tracker: compact stacktraces before printing

[Eric Dumazet]

On Thu, Feb 17, 2022 at 6:05 AM Andrzej Hajda <andrzej.hajda@xxxxxxxxx> wrote:
>
> In cases references are taken alternately on multiple exec paths leak
> report can grow substantially, sorting and grouping leaks by stack_handle
> allows to compact it.
>
> Signed-off-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> Reviewed-by: Chris Wilson <chris.p.wilson@xxxxxxxxx>
> ---
>  lib/ref_tracker.c | 35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/lib/ref_tracker.c b/lib/ref_tracker.c
> index 1b0c6d645d64a..0e9c7d2828ccb 100644
> --- a/lib/ref_tracker.c
> +++ b/lib/ref_tracker.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0-or-later
>  #include <linux/export.h>
> +#include <linux/list_sort.h>
>  #include <linux/ref_tracker.h>
>  #include <linux/slab.h>
>  #include <linux/stacktrace.h>
> @@ -14,23 +15,41 @@ struct ref_tracker {
>         depot_stack_handle_t    free_stack_handle;
>  };
>
> +static int ref_tracker_cmp(void *priv, const struct list_head *a, const struct list_head *b)
> +{
> +       const struct ref_tracker *ta = list_entry(a, const struct ref_tracker, head);
> +       const struct ref_tracker *tb = list_entry(b, const struct ref_tracker, head);
> +
> +       return ta->alloc_stack_handle - tb->alloc_stack_handle;
> +}
> +
>  void __ref_tracker_dir_print(struct ref_tracker_dir *dir,
>                            unsigned int display_limit)
>  {
> +       unsigned int i = 0, count = 0;
>         struct ref_tracker *tracker;
> -       unsigned int i = 0;
> +       depot_stack_handle_t stack;
>
>         lockdep_assert_held(&dir->lock);
>
> +       if (list_empty(&dir->list))
> +               return;
> +
> +       list_sort(NULL, &dir->list, ref_tracker_cmp);

What is going to be the cost of sorting a list with 1,000,000 items in it ?

I just want to make sure we do not trade printing at most ~10 references
(from netdev_wait_allrefs()) to a soft lockup :/ with no useful info
if something went terribly wrong.

I suggest that you do not sort a potential big list, and instead
attempt to allocate an array of @display_limits 'struct stack_counts'

I suspect @display_limits will always be kept to a reasonable value
(less than 100 ?)

struct stack_counts {
    depot_stack_handle_t stack_handle;
    unsigned int count;
}

Then, iterating the list and update the array (that you can keep
sorted by ->stack_handle)

Then after iterating, print the (at_most) @display_limits handles
found in the temp array.

> +
>         list_for_each_entry(tracker, &dir->list, head) {
> -               if (i < display_limit) {
> -                       pr_err("leaked reference.\n");
> -                       if (tracker->alloc_stack_handle)
> -                               stack_depot_print(tracker->alloc_stack_handle);
> -                       i++;
> -               } else {
> +               if (i++ >= display_limit)
>                         break;
> -               }
> +               if (!count++)
> +                       stack = tracker->alloc_stack_handle;
> +               if (stack == tracker->alloc_stack_handle &&
> +                   !list_is_last(&tracker->head, &dir->list))
> +                       continue;
> +
> +               pr_err("leaked %d references.\n", count);
> +               if (stack)
> +                       stack_depot_print(stack);
> +               count = 0;
>         }
>  }
>  EXPORT_SYMBOL(__ref_tracker_dir_print);
> --
> 2.25.1
>