linux-kernel - Re: [PATCH -v2 1/6] memcg: do not replicate try_get_mem_cgroup_from_mm in __mem_cgroup_try

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Date:	Tue, 4 Feb 2014 10:55:08 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH -v2 1/6] memcg: do not replicate
 try_get_mem_cgroup_from_mm in __mem_cgroup_try_charge

On Tue, Feb 04, 2014 at 02:28:55PM +0100, Michal Hocko wrote:
> Johannes Weiner has pointed out that __mem_cgroup_try_charge duplicates
> try_get_mem_cgroup_from_mm for charges which came without a memcg. The
> only reason seems to be a tiny optimization when css_tryget is not
> called if the charge can be consumed from the stock. Nevertheless
> css_tryget is very cheap since it has been reworked to use per-cpu
> counting so this optimization doesn't give us anything these days.
> 
> So let's drop the code duplication so that the code is more readable.
> While we are at it also remove a very confusing comment in
> try_get_mem_cgroup_from_mm.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> ---
>  mm/memcontrol.c | 49 ++++++++-----------------------------------------
>  1 file changed, 8 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 53385cd4e6f0..042e4ff36c05 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1081,11 +1081,7 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
>  
>  	if (!mm)
>  		return NULL;

While you're at it, this check also seems unnecessary.

> -	/*
> -	 * Because we have no locks, mm->owner's may be being moved to other
> -	 * cgroup. We use css_tryget() here even if this looks
> -	 * pessimistic (rather than adding locks here).
> -	 */
> +
>  	rcu_read_lock();
>  	do {
>  		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> @@ -2759,45 +2755,15 @@ again:
>  			goto done;
>  		css_get(&memcg->css);
>  	} else {
> -		struct task_struct *p;
> -
> -		rcu_read_lock();
> -		p = rcu_dereference(mm->owner);
> -		/*
> -		 * Because we don't have task_lock(), "p" can exit.
> -		 * In that case, "memcg" can point to root or p can be NULL with
> -		 * race with swapoff. Then, we have small risk of mis-accouning.
> -		 * But such kind of mis-account by race always happens because
> -		 * we don't have cgroup_mutex(). It's overkill and we allo that
> -		 * small race, here.
> -		 * (*) swapoff at el will charge against mm-struct not against
> -		 * task-struct. So, mm->owner can be NULL.
> -		 */
> -		memcg = mem_cgroup_from_task(p);
> -		if (!memcg)
> +		memcg = try_get_mem_cgroup_from_mm(mm);
> +		if (!memcg) {
>  			memcg = root_mem_cgroup;
> -		if (mem_cgroup_is_root(memcg)) {
> -			rcu_read_unlock();
> -			goto done;
> -		}
> -		if (consume_stock(memcg, nr_pages)) {
> -			/*
> -			 * It seems dagerous to access memcg without css_get().
> -			 * But considering how consume_stok works, it's not
> -			 * necessary. If consume_stock success, some charges
> -			 * from this memcg are cached on this cpu. So, we
> -			 * don't need to call css_get()/css_tryget() before
> -			 * calling consume_stock().
> -			 */
> -			rcu_read_unlock();
>  			goto done;
>  		}
> -		/* after here, we may be blocked. we need to get refcnt */
> -		if (!css_tryget(&memcg->css)) {
> -			rcu_read_unlock();
> -			goto again;
> -		}
> -		rcu_read_unlock();
> +		if (mem_cgroup_is_root(memcg))
> +			goto done_put;
> +		if (consume_stock(memcg, nr_pages))
> +			goto done_put;

These two are actually the same in the if (*ptr) branch.
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