linux-kernel - Re: [PATCH 1/6] res_counter: handle limit change

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Date:	Mon, 16 Jun 2008 11:51:04 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	kamezawa.hiroyu@...fujitsu.com
CC:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	menage@...gle.com, balbir@...ux.vnet.ibm.com,
	yamamoto@...inux.co.jp, nishimura@....nes.nec.co.jp,
	lizf@...fujitsu.com
Subject: Re: [PATCH 1/6] res_counter:  handle limit change

kamezawa.hiroyu@...fujitsu.com wrote:
> ----- Original Message -----
>>> +	 * registered callbacks etc...for res_counter.
>>> +	 */
>>> +	struct res_counter_ops ops;
>>> +	/*
> Now, write to limit is done in following path.
> sys_write() -> write_func of subsys -> write in res_counter -> 
> strategy callback -> set limit -> return
> 
> Because stragety callback is called in res_counter, we can only do 
> something after set-limit without callback. So res_counter should call
> another callback before set-limit if it can fail.
> 
>> Why would we need such? All res_counter.limit update comes via the appropiate
>> cgroup's files, so it can do whatever it needs w/o any callbacks?
>>
> 
> First reason is that this allows us to implement generic algorithm to
> handle limit change. Second is that generic algorithm can be a stack of
> functions. I don't like to pass function pointers through several stack
> of functions. (And this design allow the code to be much easier to read.
> My first version used an argument of function pointer but it was verrry ugly.)
> 
> I think when I did all in memcg, someone will comment that "why do that
> all in memcg ? please implement generic one to avoid code duplication"

Hm... But we're choosing between

sys_write->xxx_cgroup_write->res_counter_set_limit->xxx_cgroup_call

and

sys_write->xxx_cgroup_write->res_counter_set_limit
                           ->xxx_cgroup_call

With the sizeof(void *)-bytes difference in res_counter, nNo?

>> And (if we definitely need one) isn't it better to make it a
>> 	struct res_counter_ops *ops;
>> pointer?
>>
> My first version did that. When I added hierarchy_model to ops(see later patch
> ), I made use of copy of ops. But maybe you're right. Keeping 
> res_counter small is important. I'll use pointer in v5.  
> 
> Thanks,
> -Kame-
> 
> 

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