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Date:   Fri, 13 Oct 2023 16:17:21 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] mm: memcontrol: Don't css_get() on root_mem_cgroup in
 get_mem_cgroup_from_mm()
Message-ID: <ZSlRcQ3JoPLFRBec@dhcp22.suse.cz>
References: <20231012161504.3445042-1-longman@redhat.com>
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On Thu 12-10-23 12:15:04, Waiman Long wrote:
> As reference counting in the root memcg is disabled, there is no need
> to get a reference if root memcg is to be returned.

Does this give any measurable wins?
 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/memcontrol.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5b009b233ab8..2b3864194042 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1060,8 +1060,10 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>  	rcu_read_lock();
>  	do {
>  		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> -		if (unlikely(!memcg))
> +		if (unlikely(!memcg)) {
>  			memcg = root_mem_cgroup;
> +			break;
> +		}
>  	} while (!css_tryget(&memcg->css));
>  	rcu_read_unlock();
>  	return memcg;
> -- 
> 2.39.3

-- 
Michal Hocko
SUSE Labs