linux-kernel - Re: [PATCH v3 20/35] lib: add codetag reference into slabobj

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Date: Fri, 16 Feb 2024 16:36:16 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
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Subject: Re: [PATCH v3 20/35] lib: add codetag reference into slabobj_ext

On 2/12/24 22:39, Suren Baghdasaryan wrote:
> To store code tag for every slab object, a codetag reference is embedded
> into slabobj_ext when CONFIG_MEM_ALLOC_PROFILING=y.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Co-developed-by: Kent Overstreet <kent.overstreet@...ux.dev>
> Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> ---
>  include/linux/memcontrol.h | 5 +++++
>  lib/Kconfig.debug          | 1 +
>  mm/slab.h                  | 4 ++++
>  3 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index f3584e98b640..2b010316016c 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1653,7 +1653,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>   * if MEMCG_DATA_OBJEXTS is set.
>   */
>  struct slabobj_ext {
> +#ifdef CONFIG_MEMCG_KMEM
>  	struct obj_cgroup *objcg;
> +#endif
> +#ifdef CONFIG_MEM_ALLOC_PROFILING
> +	union codetag_ref ref;
> +#endif
>  } __aligned(8);

So this means that compiling with CONFIG_MEM_ALLOC_PROFILING will increase
the memory overhead of arrays allocated for CONFIG_MEMCG_KMEM, even if
allocation profiling itself is not enabled in runtime? Similar concern to
the unconditional page_ext usage, that this would hinder enabling in a
general distro kernel.

The unused field overhead would be smaller than currently page_ext, but
getting rid of it when alloc profiling is not enabled would be more work
than introducing an early boot param for the page_ext case. Could be however
solved similarly to how page_ext is populated dynamically at runtime.
Hopefully it wouldn't add noticeable cpu overhead.

>  static inline void __inc_lruvec_kmem_state(void *p, enum node_stat_item idx)
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 7bbdb0ddb011..9ecfcdb54417 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -979,6 +979,7 @@ config MEM_ALLOC_PROFILING
>  	depends on !DEBUG_FORCE_WEAK_PER_CPU
>  	select CODE_TAGGING
>  	select PAGE_EXTENSION
> +	select SLAB_OBJ_EXT
>  	help
>  	  Track allocation source code and record total allocation size
>  	  initiated at that code location. The mechanism can be used to track
> diff --git a/mm/slab.h b/mm/slab.h
> index 77cf7474fe46..224a4b2305fb 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -569,6 +569,10 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  
>  static inline bool need_slab_obj_ext(void)
>  {
> +#ifdef CONFIG_MEM_ALLOC_PROFILING
> +	if (mem_alloc_profiling_enabled())
> +		return true;
> +#endif
>  	/*
>  	 * CONFIG_MEMCG_KMEM creates vector of obj_cgroup objects conditionally
>  	 * inside memcg_slab_post_alloc_hook. No other users for now.