linux-kernel - Re: [PATCH v3 15/19] mm: memcg/slab: deprecate slab_root

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Message-ID: <20200526185027.GD377498@carbon.DHCP.thefacebook.com>
Date:   Tue, 26 May 2020 11:50:27 -0700
From:   Roman Gushchin <guro@...com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 15/19] mm: memcg/slab: deprecate slab_root_caches

On Tue, May 26, 2020 at 12:52:24PM +0200, Vlastimil Babka wrote:
> On 4/22/20 10:47 PM, Roman Gushchin wrote:
> > Currently there are two lists of kmem_caches:
> > 1) slab_caches, which contains all kmem_caches,
> > 2) slab_root_caches, which contains only root kmem_caches.
> > 
> > And there is some preprocessor magic to have a single list
> > if CONFIG_MEMCG_KMEM isn't enabled.
> > 
> > It was required earlier because the number of non-root kmem_caches
> > was proportional to the number of memory cgroups and could reach
> > really big values. Now, when it cannot exceed the number of root
> > kmem_caches, there is really no reason to maintain two lists.
> > 
> > We never iterate over the slab_root_caches list on any hot paths,
> > so it's perfectly fine to iterate over slab_caches and filter out
> > non-root kmem_caches.
> > 
> > It allows to remove a lot of config-dependent code and two pointers
> > from the kmem_cache structure.
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> 
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

> 
> > @@ -1148,11 +1126,12 @@ static void cache_show(struct kmem_cache *s, struct seq_file *m)
> >  
> >  static int slab_show(struct seq_file *m, void *p)
> >  {
> > -	struct kmem_cache *s = list_entry(p, struct kmem_cache, root_caches_node);
> > +	struct kmem_cache *s = list_entry(p, struct kmem_cache, list);
> >  
> > -	if (p == slab_root_caches.next)
> > +	if (p == slab_caches.next)
> >  		print_slabinfo_header(m);
> > -	cache_show(s, m);
> > +	if (is_root_cache(s))
> > +		cache_show(s, m);
> 
> If there wasn't patch 17/19 we could just remove this condition and have
> /proc/slabinfo contain the -memcg variants?

Sure, it's an option too. But because it's a user facing option, I'd keep it
as it is now at least until everything will settle down a bit.

Thanks!