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Message-ID: <CANpmjNOYwy4h9zWJVFANO6geakvcyccppNfFjs7qwSwmLpFLYQ@mail.gmail.com>
Date: Mon, 21 Jul 2025 12:26:57 +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 v4] stackdepot: Make max number of pools boot-time configurable
On Fri, 18 Jul 2025 at 17:39, 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 because we have long stacks that don't dedup very well.
>
> Introduce a new start-up parameter to allow users to set the number of
> maximum stack depot pools.
>
> Signed-off-by: Matt Fleming <mfleming@...udflare.com>
Acked-by: Marco Elver <elver@...gle.com>
Minor improvement suggestions below (at your discretion).
Thanks!
> ---
>
> Changes in v4:
> - Simplify initial stack_max_pools value courtesy of akpm
>
> Changes in v3:
> - Switch from build-time to boot-time parameter
>
> Changes in v2:
> - Replace BUILD_BUG_ON with static_assert()
> - Hide STACKDEPOT_MAX_POOLS behind EXPERT
>
> .../admin-guide/kernel-parameters.txt | 5 ++
> lib/stackdepot.c | 67 ++++++++++++++++---
> 2 files changed, 63 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 07e22ba5bfe3..273f6bbd56c2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6988,6 +6988,11 @@
> consumed by the stack hash table. By default this is set
> to false.
>
> + stack_depot_max_pools= [KNL,EARLY]
> + Specify the maximum number of pools to use for storing
> + stack traces. Pools are allocated on-demand up to this
> + limit. Default value is 8191 pools.
> +
> stacktrace [FTRACE]
> Enabled the stack tracer on boot up.
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 73d7b50924ef..de0b0025af2b 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -36,11 +36,11 @@
> #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)
> +/*
> + * The pool_index is offset by 1 so the first record does not have a 0 handle.
> + */
> +static unsigned int stack_max_pools __read_mostly =
This could be __ro_after_init.
> + MIN((1LL << DEPOT_POOL_INDEX_BITS) - 1, 8192);
>
> static bool stack_depot_disabled;
> static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> @@ -62,7 +62,7 @@ static unsigned int stack_bucket_number_order;
> static unsigned int stack_hash_mask;
>
> /* Array of memory regions that store stack records. */
> -static void *stack_pools[DEPOT_MAX_POOLS];
> +static void **stack_pools;
> /* Newly allocated pool that is not yet added to stack_pools. */
> static void *new_pool;
> /* Number of pools in stack_pools. */
> @@ -101,6 +101,34 @@ static int __init disable_stack_depot(char *str)
> }
> early_param("stack_depot_disable", disable_stack_depot);
>
> +static int __init parse_max_pools(char *str)
> +{
> + const long long limit = (1LL << (DEPOT_POOL_INDEX_BITS)) - 1;
> + unsigned int max_pools;
> + int rv;
> +
> + rv = kstrtouint(str, 0, &max_pools);
> + if (rv)
> + return rv;
> +
> + if (max_pools < 1024) {
> + pr_err("stack_depot_max_pools below 1024, using default of %u\n",
> + stack_max_pools);
nit: I feel the choice of 1024 should be somewhat more documented,
because right now it looks like a magic value.
> + goto out;
> + }
> +
> + if (max_pools > limit) {
> + pr_err("stack_depot_max_pools exceeds %lld, using default of %u\n",
> + limit, stack_max_pools);
> + goto out;
> + }
> +
> + stack_max_pools = max_pools;
> +out:
> + return 0;
> +}
> +early_param("stack_depot_max_pools", parse_max_pools);
> +
> void __init stack_depot_request_early_init(void)
> {
> /* Too late to request early init now. */
> @@ -182,6 +210,17 @@ int __init stack_depot_early_init(void)
> }
> init_stack_table(entries);
>
> + pr_info("allocating space for %u stack pools via memblock\n",
> + stack_max_pools);
> + stack_pools =
> + memblock_alloc(stack_max_pools * sizeof(void *), PAGE_SIZE);
Bike-shedding territory: Kernel allows up to 120 cols if it helps
readability. Line-break-assignments just look wrong and clang-format
should be fixed. Regardless, I know some folks just want to follow
what clang-format suggests without much consideration for readability.
> + if (!stack_pools) {
> + pr_err("stack pools allocation failed, disabling\n");
> + memblock_free(stack_table, entries * sizeof(struct list_head));
> + stack_depot_disabled = true;
> + return -ENOMEM;
> + }
> +
> return 0;
> }
>
> @@ -231,6 +270,16 @@ int stack_depot_init(void)
> stack_hash_mask = entries - 1;
> init_stack_table(entries);
>
> + pr_info("allocating space for %u stack pools via kvcalloc\n",
> + stack_max_pools);
> + stack_pools = kvcalloc(stack_max_pools, sizeof(void *), GFP_KERNEL);
> + if (!stack_pools) {
> + pr_err("stack pools allocation failed, disabling\n");
> + kvfree(stack_table);
> + stack_depot_disabled = true;
> + ret = -ENOMEM;
> + }
> +
> out_unlock:
> mutex_unlock(&stack_depot_init_mutex);
>
> @@ -245,9 +294,9 @@ static bool depot_init_pool(void **prealloc)
> {
> lockdep_assert_held(&pool_lock);
>
> - if (unlikely(pools_num >= DEPOT_MAX_POOLS)) {
> + if (unlikely(pools_num >= stack_max_pools)) {
> /* Bail out if we reached the pool limit. */
> - WARN_ON_ONCE(pools_num > DEPOT_MAX_POOLS); /* should never happen */
> + WARN_ON_ONCE(pools_num > stack_max_pools); /* should never happen */
> WARN_ON_ONCE(!new_pool); /* to avoid unnecessary pre-allocation */
> WARN_ONCE(1, "Stack depot reached limit capacity");
> return false;
> @@ -273,7 +322,7 @@ static bool depot_init_pool(void **prealloc)
> * NULL; do not reset to NULL if we have reached the maximum number of
> * pools.
> */
> - if (pools_num < DEPOT_MAX_POOLS)
> + if (pools_num < stack_max_pools)
> WRITE_ONCE(new_pool, NULL);
> else
> WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
> --
> 2.34.1
>
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