linux-kernel - Re: [PATCH] memcg: replace memcg ID idr with xarray

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Message-ID: <ZrnCt_cGOOwQxWn3@tiehlicka>
Date: Mon, 12 Aug 2024 10:07:19 +0200
From: Michal Hocko <mhocko@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>, cgroups@...r.kernel.org,
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] memcg: replace memcg ID idr with xarray

On Fri 09-08-24 10:26:18, Shakeel Butt wrote:
> At the moment memcg IDs are managed through IDR which requires external
> synchronization mechanisms and makes the allocation code a bit awkward.
> Let's switch to xarray and make the code simpler.
> 
> Suggested-by: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Neat! I was not aware of this feature of XArray. Is there any actual
reason for idr to have its own implementation with XArray offering a
better interface?

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  mm/memcontrol.c | 34 +++++++---------------------------
>  1 file changed, 7 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e1ffd2950393..b8e6b98485c6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3363,29 +3363,12 @@ static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
>   */
>  
>  #define MEM_CGROUP_ID_MAX	((1UL << MEM_CGROUP_ID_SHIFT) - 1)
> -static DEFINE_IDR(mem_cgroup_idr);
> -static DEFINE_SPINLOCK(memcg_idr_lock);
> -
> -static int mem_cgroup_alloc_id(void)
> -{
> -	int ret;
> -
> -	idr_preload(GFP_KERNEL);
> -	spin_lock(&memcg_idr_lock);
> -	ret = idr_alloc(&mem_cgroup_idr, NULL, 1, MEM_CGROUP_ID_MAX + 1,
> -			GFP_NOWAIT);
> -	spin_unlock(&memcg_idr_lock);
> -	idr_preload_end();
> -	return ret;
> -}
> +static DEFINE_XARRAY_ALLOC1(mem_cgroup_ids);
>  
>  static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
>  {
>  	if (memcg->id.id > 0) {
> -		spin_lock(&memcg_idr_lock);
> -		idr_remove(&mem_cgroup_idr, memcg->id.id);
> -		spin_unlock(&memcg_idr_lock);
> -
> +		xa_erase(&mem_cgroup_ids, memcg->id.id);
>  		memcg->id.id = 0;
>  	}
>  }
> @@ -3420,7 +3403,7 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
>  struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
>  {
>  	WARN_ON_ONCE(!rcu_read_lock_held());
> -	return idr_find(&mem_cgroup_idr, id);
> +	return xa_load(&mem_cgroup_ids, id);
>  }
>  
>  #ifdef CONFIG_SHRINKER_DEBUG
> @@ -3519,11 +3502,10 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
>  	if (!memcg)
>  		return ERR_PTR(error);
>  
> -	memcg->id.id = mem_cgroup_alloc_id();
> -	if (memcg->id.id < 0) {
> -		error = memcg->id.id;
> +	error = xa_alloc(&mem_cgroup_ids, &memcg->id.id, NULL,
> +			 XA_LIMIT(1, MEM_CGROUP_ID_MAX), GFP_KERNEL);
> +	if (error)
>  		goto fail;
> -	}
>  
>  	memcg->vmstats = kzalloc(sizeof(struct memcg_vmstats),
>  				 GFP_KERNEL_ACCOUNT);
> @@ -3664,9 +3646,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>  	 * publish it here at the end of onlining. This matches the
>  	 * regular ID destruction during offlining.
>  	 */
> -	spin_lock(&memcg_idr_lock);
> -	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> -	spin_unlock(&memcg_idr_lock);
> +	xa_store(&mem_cgroup_ids, memcg->id.id, memcg, GFP_KERNEL);
>  
>  	return 0;
>  offline_kmem:
> -- 
> 2.43.5
> 

-- 
Michal Hocko
SUSE Labs