linux-kernel - Re: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID

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Date:	Fri, 16 Jan 2009 09:49:05 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID

KAMEZAWA Hiroyuki wrote:
> On Fri, 16 Jan 2009 09:29:48 +0800
> Li Zefan <lizf@...fujitsu.com> wrote:
> 
>>>  /*
>>> - * Dance down the hierarchy if needed to reclaim memory. We remember the
>>> - * last child we reclaimed from, so that we don't end up penalizing
>>> - * one child extensively based on its position in the children list.
>>> + * Visit the first child (need not be the first child as per the ordering
>>> + * of the cgroup list, since we track last_scanned_child) of @mem and use
>>> + * that to reclaim free pages from.
>>> + */
>>> +static struct mem_cgroup *
>>> +mem_cgroup_select_victim(struct mem_cgroup *root_mem)
>>> +{
>>> +	struct mem_cgroup *ret = NULL;
>>> +	struct cgroup_subsys_state *css;
>>> +	int nextid, found;
>>> +
>>> +	if (!root_mem->use_hierarchy) {
>>> +		spin_lock(&root_mem->reclaim_param_lock);
>>> +		root_mem->scan_age++;
>>> +		spin_unlock(&root_mem->reclaim_param_lock);
>>> +		css_get(&root_mem->css);
>>> +		ret = root_mem;
>>> +	}
>>> +
>>> +	while (!ret) {
>>> +		rcu_read_lock();
>>> +		nextid = root_mem->last_scanned_child + 1;
>>> +		css = css_get_next(&mem_cgroup_subsys, nextid, &root_mem->css,
>>> +				   &found);
>>> +		if (css && css_is_populated(css) && css_tryget(css))
>> I don't see why you need to check css_is_populated(css) ?
>>
> 
> Main reason is for sanity. I don't like to hold css->refcnt of not populated css.

I think this is a rare case. It's just a very short period when a cgroup is
being created but not yet fully created.

> Second reason is for avoinding unnecessary calls to try_to_free_pages(),
> it's heavy. I should also add mem->res.usage == 0 case for skipping but not yet.
> 

And if mem->res.usage == 0 is checked, css_is_popuated() is just redundant.

> THanks,
> -Kame
> 
>>> +			ret = container_of(css, struct mem_cgroup, css);
>>> +
>>> +		rcu_read_unlock();
>>> +		/* Updates scanning parameter */
>>> +		spin_lock(&root_mem->reclaim_param_lock);
>>> +		if (!css) {
>>> +			/* this means start scan from ID:1 */
>>> +			root_mem->last_scanned_child = 0;
>>> +			root_mem->scan_age++;
>>> +		} else
>>> +			root_mem->last_scanned_child = found;
>>> +		spin_unlock(&root_mem->reclaim_param_lock);
>>> +	}
>>> +
>>> +	return ret;
>>> +}
>>> +
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