Re: [PATCH] lib: stackdepot: Add support to configure STACK_HASH_SIZE

[Minchan Kim <minchan@kernel.org>]

Sorry if this mail corrupts the mail thread or had heavy mangling
since I lost this mail from my mailbox so I am sending this mail by
web gmail.

On Thu, Oct 22, 2020 at 10:18 AM <vjitta@codeaurora.org> wrote:
>
> From: Yogesh Lal <ylal@codeaurora.org>
>
> Use STACK_HASH_ORDER_SHIFT to configure STACK_HASH_SIZE.
>
> Aim is to have configurable value for  STACK_HASH_SIZE,
> so depend on use case one can configure it.
>
> One example is of Page Owner, default value of
> STACK_HASH_SIZE lead stack depot to consume 8MB of static memory.
> Making it configurable and use lower value helps to enable features like
> CONFIG_PAGE_OWNER without any significant overhead.
>
> Signed-off-by: Yogesh Lal <ylal@codeaurora.org>
> Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
> Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
> ---
>  lib/Kconfig      | 9 +++++++++
>  lib/stackdepot.c | 3 +--
>  2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 18d76b6..b3f8259 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -651,6 +651,15 @@ config STACKDEPOT
>         bool
>         select STACKTRACE
>
> +config STACK_HASH_ORDER_SHIFT
> +       int "stack depot hash size (12 => 4KB, 20 => 1024KB)"
> +       range 12 20
> +       default 20
> +       depends on STACKDEPOT
> +       help
> +        Select the hash size as a power of 2 for the stackdepot hash table.
> +        Choose a lower value to reduce the memory impact.
> +
>  config SBITMAP
>         bool
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 2caffc6..413c20b 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -142,8 +142,7 @@ static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
>         return stack;
>  }
>
> -#define STACK_HASH_ORDER 20
> -#define STACK_HASH_SIZE (1L << STACK_HASH_ORDER)
> +#define STACK_HASH_SIZE (1L << CONFIG_STACK_HASH_ORDER_SHIFT)
>  #define STACK_HASH_MASK (STACK_HASH_SIZE - 1)
>  #define STACK_HASH_SEED 0x9747b28c
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
> 2.7.4
>

1. When we don't use page_owner, we don't want to waste any memory for
stackdepot hash array.
2. When we use page_owner, we want to have reasonable stackdeport hash array

With this configuration, it couldn't meet since we always need to
reserve a reasonable size for the array.
Can't we make the hash size as a kernel parameter?
With it, we could use it like this.

1. page_owner=off, stackdepot_stack_hash=0 -> no more wasted memory
when we don't use page_owner
2. page_owner=on, stackdepot_stack_hash=8M -> reasonable hash size
when we use page_owner.


-- 
Kind regards,
Minchan Kim