linux-kernel - Re: [RFC][PATCH 2/5] add softlimit to res

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Date:	Thu, 12 Mar 2009 09:40:38 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.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>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC][PATCH 2/5] add softlimit to res_counter

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-12 12:58:39]:

> On Thu, 12 Mar 2009 09:24:44 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> >
> > > +int res_counter_set_softlimit(struct res_counter *cnt, unsigned long long val)
> > > +{
> > > +	unsigned long flags;
> > > +
> > > +	spin_lock_irqsave(&cnt->lock, flags);
> > > +	cnt->softlimit = val;
> > > +	spin_unlock_irqrestore(&cnt->lock, flags);
> > > +	return 0;
> > > +}
> > > +
> > > +bool res_counter_check_under_softlimit(struct res_counter *cnt)
> > > +{
> > > +	struct res_counter *c;
> > > +	unsigned long flags;
> > > +	bool ret = true;
> > > +
> > > +	local_irq_save(flags);
> > > +	for (c = cnt; ret && c != NULL; c = c->parent) {
> > > +		spin_lock(&c->lock);
> > > +		if (c->softlimit < c->usage)
> > > +			ret = false;
> > 
> > So if a child was under the soft limit and the parent is *not*, we
> > _override_ ret and return false?
> > 
> yes. If you don't want this behavior I'll rename this to
> res_counter_check_under_softlimit_hierarchical().
> 

That is a nicer name.

> 
> > > +		spin_unlock(&c->lock);
> > > +	}
> > > +	local_irq_restore(flags);
> > > +	return ret;
> > > +}
> > 
> > Why is the check_under_softlimit hierarchical? 
> 
> At checking whether a mem_cgroup is a candidate for softlimit-reclaim,
> we need to check all parents.
> 
> > BTW, this patch is buggy. See above.
> > 
> 
> Not buggy. Just meets my requiremnt.

Correct me if I am wrong, but this boils down to checking if the top
root is above it's soft limit? Instead of checking all the way up in
the hierarchy, can't we do a conditional check for

        c->parent == NULL && (c->softlimit < c->usage)

BTW, I would prefer to split the word softlimit to soft_limit, it is
more readable that way.


-- 
	Balbir
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