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

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Date:	Tue, 3 Mar 2009 09:03:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.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)

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.


Thanks,
-Kame






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