Re: [PATCH 1/2] alloc_tag: avoid current->alloc_tag manipulations when profiling is disabled

[Kent Overstreet <kent.overstreet@linux.dev>]

On Thu, Dec 26, 2024 at 01:16:38PM -0800, Suren Baghdasaryan wrote:
> When memory allocation profiling is disabled there is no need to update
> current->alloc_tag and these manipulations add unnecessary overhead. Fix
> the overhead by skipping these extra updates.

I did it the other way because I was concerned about the overhead of
adding a huge number of static keys. But on further thought a static key
probably isn't any bigger than an alloc tag, no?

> 
> Fixes: b951aaff5035 ("mm: enable page allocation tagging")
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Cc: stable@vger.kernel.org
> ---
>  include/linux/alloc_tag.h | 11 ++++++++---
>  lib/alloc_tag.c           |  2 ++
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> index 0bbbe537c5f9..a946e0203e6d 100644
> --- a/include/linux/alloc_tag.h
> +++ b/include/linux/alloc_tag.h
> @@ -224,9 +224,14 @@ static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {}
>  
>  #define alloc_hooks_tag(_tag, _do_alloc)				\
>  ({									\
> -	struct alloc_tag * __maybe_unused _old = alloc_tag_save(_tag);	\
> -	typeof(_do_alloc) _res = _do_alloc;				\
> -	alloc_tag_restore(_tag, _old);					\
> +	typeof(_do_alloc) _res;						\
> +	if (mem_alloc_profiling_enabled()) {				\
> +		struct alloc_tag * __maybe_unused _old;			\
> +		_old = alloc_tag_save(_tag);				\
> +		_res = _do_alloc;					\
> +		alloc_tag_restore(_tag, _old);				\
> +	} else								\
> +		_res = _do_alloc;					\
>  	_res;								\
>  })
>  
> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> index 7dcebf118a3e..4c373f444eb1 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -29,6 +29,8 @@ EXPORT_SYMBOL(_shared_alloc_tag);
>  
>  DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>  			mem_alloc_profiling_key);
> +EXPORT_SYMBOL(mem_alloc_profiling_key);
> +
>  DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
>  
>  struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
> 
> base-commit: 431614f1580a03c1a653340c55ea76bd12a9403f
> -- 
> 2.47.1.613.gc27f4b7a9f-goog
>