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Subject: Re: [PATCH v3 08/19] mm: memcg/slab: save obj_cgroup for non-root
 slab objects
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
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From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <90706966-b64d-751d-5f5a-b1e72fbfb30a@suse.cz>
Date:   Wed, 27 May 2020 13:03:17 +0200
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On 5/26/20 7:53 PM, Roman Gushchin wrote:
> On Mon, May 25, 2020 at 05:07:22PM +0200, Vlastimil Babka wrote:
>> On 4/22/20 10:46 PM, Roman Gushchin wrote:
>> > diff --git a/mm/slab.h b/mm/slab.h
>> > index 44def57f050e..525e09e05743 100644
>> > --- a/mm/slab.h
>> > +++ b/mm/slab.h
>> ...
>> > @@ -636,8 +684,8 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
>> >  					 s->flags, flags);
>> >  	}
>> >  
>> > -	if (memcg_kmem_enabled())
>> > -		memcg_kmem_put_cache(s);
>> > +	if (!is_root_cache(s))
>> > +		memcg_slab_post_alloc_hook(s, objcg, size, p);
>> >  }
>> >  
>> >  #ifndef CONFIG_SLOB
>> 
>> Keep also the memcg_kmem_enabled() static key check, like elsewhere?
>> 
> 
> Ok, will add, it can speed things up a little bit. My only concern is that
> the code is not ready for memcg_kmem_enabled() turning negative after being positive.
> But it's not a concern, right?
> 
> Actually, we can simplify memcg_kmem_enabled() mechanics and enable it
> only once as soon as the first memcg is fully initialized. I don't think there
> is any value in tracking the actual number of active memcgs.

Yeah, it should be acceptable that once the key is enabled after boot, there's
no way back until reboot.

> Thanks!
>