Re: [PATCH] mm, memcg: reduce size of struct mem_cgroup by using bit field

[Michal Hocko <mhocko@kernel.org>]

On Fri 27-12-19 07:43:52, Yafang Shao wrote:
> There are some members in struct mem_group can be either 0(false) or
> 1(true), so we can define them using bit field to reduce size. With this
> patch, the size of struct mem_cgroup can be reduced by 64 bytes in theory,
> but as there're some MEMCG_PADDING()s, the real number may be different,
> which is relate with the cacheline size. Anyway, this patch could reduce
> the size of struct mem_cgroup more or less.

Pahole says
        /* size: 2496, cachelines: 39, members: 48 */
        /* sum members: 2364, holes: 8, sum holes: 108 */
        /* padding: 24 */
        /* forced alignments: 3, forced holes: 2, sum forced holes: 88 */

after the change
        /* size: 2496, cachelines: 39, members: 48 */
        /* sum members: 2352, holes: 7, sum holes: 108 */
        /* sum bitfield members: 6 bits, bit holes: 1, sum bit holes: 26 bits */
        /* padding: 32 */
        /* forced alignments: 3, forced holes: 2, sum forced holes: 88 */

so exactly 0 change. I do not think this reshuffling is worth it.

> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Roman Gushchin <guro@fb.com>
> ---
>  include/linux/memcontrol.h | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index a7a0a1a5..f68a9ef 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -229,20 +229,26 @@ struct mem_cgroup {
>  	/*
>  	 * Should the accounting and control be hierarchical, per subtree?
>  	 */
> -	bool use_hierarchy;
> +	unsigned int use_hierarchy : 1;
> +
> +	/* Legacy tcp memory accounting */
> +	unsigned int tcpmem_active : 1;
> +	unsigned int tcpmem_pressure : 1;
>  
>  	/*
>  	 * Should the OOM killer kill all belonging tasks, had it kill one?
>  	 */
> -	bool oom_group;
> +	unsigned int  oom_group : 1;
>  
>  	/* protected by memcg_oom_lock */
> -	bool		oom_lock;
> -	int		under_oom;
> +	unsigned int oom_lock : 1;
>  
> -	int	swappiness;
>  	/* OOM-Killer disable */
> -	int		oom_kill_disable;
> +	unsigned int oom_kill_disable : 1;
> +
> +	int under_oom;
> +
> +	int	swappiness;
>  
>  	/* memory.events and memory.events.local */
>  	struct cgroup_file events_file;
> @@ -297,9 +303,6 @@ struct mem_cgroup {
>  
>  	unsigned long		socket_pressure;
>  
> -	/* Legacy tcp memory accounting */
> -	bool			tcpmem_active;
> -	int			tcpmem_pressure;
>  
>  #ifdef CONFIG_MEMCG_KMEM
>          /* Index in the kmem_cache->memcg_params.memcg_caches array */
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs