linux-kernel - Re: [PATCH 0/4] Memory controller soft limit patches (v3)

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Date:	Tue, 3 Mar 2009 16:53:06 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Bharata B Rao <bharata@...ibm.com>,
	Paul Menage <menage@...gle.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] Memory controller soft limit patches (v3)

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-03 09:03:03]:

> On Mon, 2 Mar 2009 23:22:35 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-02 15:18:30]:
> > 
> > > On Mon, 2 Mar 2009 11:35:19 +0530
> > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > > 
> > > > > Then, not-sorted RB-tree can be there.
> > > > > 
> > > > > BTW,
> > > > >    time_after(jiffies, 0)
> > > > > is buggy (see definition). If you want make this true always,
> > > > >    time_after(jiffies, jiffies +1)
> > > > >
> > > > 
> > > > HZ/4 is 250/4 jiffies in the worst case (62). We have
> > > > time_after(jiffies, next_update_interval) and next_update_interval is
> > > > set to last_tree_update + 62. Not sure if I got what you are pointing
> > > > to.
> > > > 
> > > +	unsigned long next_update = 0;
> > > +	unsigned long flags;
> > > +
> > > +	if (!css_tryget(&mem->css))
> > > +		return;
> > > +	prev_usage_in_excess = mem->usage_in_excess;
> > > +	new_usage_in_excess = res_counter_soft_limit_excess(&mem->res);
> > > +
> > > +	if (time_check)
> > > +		next_update = mem->last_tree_update +
> > > +				MEM_CGROUP_TREE_UPDATE_INTERVAL;
> > > +	if (new_usage_in_excess && time_after(jiffies, next_update)) {
> > > +		if (prev_usage_in_excess)
> > > +			mem_cgroup_remove_exceeded(mem);
> > > +		mem_cgroup_insert_exceeded(mem);
> > > +		updated_tree = true;
> > > +	} else if (prev_usage_in_excess && !new_usage_in_excess) {
> > > +		mem_cgroup_remove_exceeded(mem);
> > > +		updated_tree = true;
> > > +	}
> > > 
> > > My point is what happens if time_check==false.
> > > time_afrter(jiffies, 0) is buggy.
> > >
> > 
> > I see your point now, but the idea behind doing so is that
> > time_after(jiffies, 0) will always return false, which forces the
> > prev_usage_in_excess and !new_usage_in_excess check to execute. We set
> > the value to false only from __mem_cgroup_free().
> > 
> > Are you suggesting that calling time_after(jiffies, 0) is buggy?
> > The comment
> > 
> >   Do this with "<0" and ">=0" to only test the sign of the result. A
> >  
> > I think refers to the comparison check and not to the parameters. I
> > hope I am reading this right.
> 
>  106 #define time_after(a,b)         \
>  107         (typecheck(unsigned long, a) && \
>  108          typecheck(unsigned long, b) && \
>  109          ((long)(b) - (long)(a) < 0))
> 
> Reading above.
> 
>   if b==0.
>      if (long)a <0  -> false
>      if (long)a >0  -> true
> 
> jiffies is unsigned value. please think of bit-pattern of signed/unsigned value.

Fair enough, the cast to long will be an issue. I'll fix it.

-- 
	Balbir
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