Re: [PATCH -mmotm 5/7] memcg: avoid oom during moving charge

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 4 Dec 2009 14:51:54 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> This move-charge-at-task-migration feature has extra charges on "to"(pre-charges)
> and "from"(leftover charges) during moving charge. This means unnecessary oom
> can happen.
> 
> This patch tries to avoid such oom.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> Changelog: 2009/12/04
> - take account of "from" too, because we uncharge from "from" at once in
>   mem_cgroup_clear_mc(), so leftover charges exist during moving charge.
> - check use_hierarchy of "mem_over_limit", instead of "to" or "from"(bugfix).
> ---
>  mm/memcontrol.c |   38 ++++++++++++++++++++++++++++++++++++++
>  1 files changed, 38 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 769b85a..f50ad15 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -253,6 +253,7 @@ struct move_charge_struct {
>  	struct mem_cgroup *to;
>  	unsigned long precharge;
>  	unsigned long moved_charge;
> +	struct task_struct *moving_task;	/* a task moving charges */
>  };
>  static struct move_charge_struct mc;
>  
> @@ -1504,6 +1505,40 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
>  		if (mem_cgroup_check_under_limit(mem_over_limit))
>  			continue;
>  
> +		/* try to avoid oom while someone is moving charge */
> +		if (mc.moving_task && current != mc.moving_task) {
> +			struct mem_cgroup *from, *to;
> +			bool do_continue = false;
> +			/*
> +			 * There is a small race that "from" or "to" can be
> +			 * freed by rmdir, so we use css_tryget().
> +			 */
> +			rcu_read_lock();
> +			from = mc.from;
> +			to = mc.to;
> +			if (from && css_tryget(&from->css)) {
> +				if (mem_over_limit->use_hierarchy)
> +					do_continue = css_is_ancestor(
> +							&from->css,
> +							&mem_over_limit->css);
> +				else
> +					do_continue = (from == mem_over_limit);
> +				css_put(&from->css);
> +			}
> +			if (!do_continue && to && css_tryget(&to->css)) {
> +				if (mem_over_limit->use_hierarchy)
> +					do_continue = css_is_ancestor(
> +							&to->css,
> +							&mem_over_limit->css);
> +				else
> +					do_continue = (to == mem_over_limit);
> +				css_put(&to->css);
> +			}
> +			rcu_read_unlock();
> +			if (do_continue)
> +				continue;

Hmm. do countine without any relaxing ? can't this occupy cpu ?
Can't we add schedule() or some and put into sleep ?

Maybe the best way is enqueue this thread to mc.wait_queue and wait for
the end of task moving.

Thanks,
-Kame

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