linux-kernel - Re: [PATCH] stackdepot: Make max number of pools build-time configurable

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Message-ID: <CANpmjNMkVGVnmeKK5QmwDPZv9QrF0omNz-TmgY0Dqqg-=uAN4Q@mail.gmail.com>
Date: Tue, 10 Jun 2025 13:35:44 +0200
From: Marco Elver <elver@...gle.com>
To: Matt Fleming <matt@...dmodwrite.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	kernel-team@...udflare.com, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Oscar Salvador <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>, 
	Matt Fleming <mfleming@...udflare.com>
Subject: Re: [PATCH] stackdepot: Make max number of pools build-time configurable

On Tue, 10 Jun 2025 at 13:16, Matt Fleming <matt@...dmodwrite.com> wrote:
>
> From: Matt Fleming <mfleming@...udflare.com>
>
> We're hitting the WARN in depot_init_pool() about reaching the stack
> depot limit. My assumption is because we have long stacks that don't
> dedup very well.
>
> Introduce a new config to allow users to set the number of maximum stack
> depot pools at build time. Also, turn the silent capping into a
> build-time assert to provide more obvious feedback when users set this
> value too high.
>
> Signed-off-by: Matt Fleming <mfleming@...udflare.com>
> ---
>  lib/Kconfig      | 6 ++++++
>  lib/stackdepot.c | 9 ++++-----
>  2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/lib/Kconfig b/lib/Kconfig
> index b38849af6f13..2c5af89daff9 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -720,6 +720,12 @@ config STACKDEPOT_MAX_FRAMES
>         default 64
>         depends on STACKDEPOT
>
> +config STACKDEPOT_MAX_POOLS
> +       int "Maximum number of stack depot pools to store stack traces"

I wouldn't want most kernel configs change this. Can we make it an
expert option? I.e.

  int "Maximum number of stack depot pools to store stack traces" if EXPERT

> +       range 1024 131071
> +       default 8192
> +       depends on STACKDEPOT
> +
>  config REF_TRACKER
>         bool
>         depends on STACKTRACE_SUPPORT
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 245d5b416699..1c24230b4a37 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -36,11 +36,7 @@
>  #include <linux/memblock.h>
>  #include <linux/kasan-enabled.h>
>
> -#define DEPOT_POOLS_CAP 8192
> -/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> -#define DEPOT_MAX_POOLS \
> -       (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> -        (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> +#define DEPOT_MAX_POOLS CONFIG_STACKDEPOT_MAX_POOLS
>
>  static bool stack_depot_disabled;
>  static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> @@ -245,6 +241,9 @@ static bool depot_init_pool(void **prealloc)
>  {
>         lockdep_assert_held(&pool_lock);
>
> +       /* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> +       BUILD_BUG_ON((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_MAX_POOLS);

This is a constant expression, and could be a static_assert() (with
inverted condition) next to the '#define DEPOT_MAX_POOLS'.
BUILD_BUG_ON() is a bit less efficient (compile-time wise), and cannot
be substituted with static_assert() only if part of the expression is
a non-constant expression yet we rely on the compiler to optimize the
condition into a constant if there's a bug.

>         if (unlikely(pools_num >= DEPOT_MAX_POOLS)) {
>                 /* Bail out if we reached the pool limit. */
>                 WARN_ON_ONCE(pools_num > DEPOT_MAX_POOLS); /* should never happen */
> --
> 2.34.1
>