linux-kernel - Re: [PATCH v3 1/2] mm: memcg: remote memcg charging for kmem allocations

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Date:   Tue, 13 Mar 2018 14:49:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: memcg: remote memcg charging for kmem
 allocations

On Wed 21-02-18 14:37:56, Shakeel Butt wrote:
[...]
> +#ifdef CONFIG_MEMCG
> +static inline struct mem_cgroup *memalloc_memcg_save(struct mem_cgroup *memcg)
> +{
> +	struct mem_cgroup *old_memcg = current->target_memcg;
> +	current->target_memcg = memcg;
> +	return old_memcg;
> +}

So you are relying that the caller will handle the reference counting
properly? I do not think this is a good idea. Also do we need some kind
of debugging facility to detect unbalanced save/restore scopes?

[...]
> @@ -2260,7 +2269,10 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>  	if (current->memcg_kmem_skip_account)
>  		return cachep;
>  
> -	memcg = get_mem_cgroup_from_mm(current->mm);
> +	if (current->target_memcg)
> +		memcg = get_mem_cgroup(current->target_memcg);
> +	if (!memcg)
> +		memcg = get_mem_cgroup_from_mm(current->mm);
>  	kmemcg_id = READ_ONCE(memcg->kmemcg_id);
>  	if (kmemcg_id < 0)
>  		goto out;

You are also adding one branch for _each_ charge path even though the
usecase is rather limited.

I will have to think about this approach more. It is clearly less code
than your previous attempt but I cannot say I would be really impressed.
-- 
Michal Hocko
SUSE Labs