linux-kernel - Re: [ckrm-tech] [PATCH 3/9] Containers (V9): Add tasks file interface

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Message-ID: <46380C49.3070404@linux.vnet.ibm.com>
Date:	Wed, 02 May 2007 09:28:01 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
CC:	vatsa@...ibm.com, ckrm-tech@...ts.sourceforge.net,
	balbir@...ibm.com, haveblue@...ibm.com, xemul@...ru, dev@...ru,
	containers@...ts.osdl.org, pj@....com, devel@...nvz.org,
	ebiederm@...ssion.com, mbligh@...gle.com, rohitseth@...gle.com,
	serue@...ibm.com, akpm@...ux-foundation.org,
	svaidy@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [ckrm-tech] [PATCH 3/9] Containers (V9): Add tasks file interface

Paul Menage wrote:
> On 5/1/07, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>> > +     if (container_is_removed(cont)) {
>> > +             retval = -ENODEV;
>> > +             goto out2;
>> > +     }
>>
>> Can't we make this check prior to kmalloc() and copy_from_user()?
> 
> We could but I'm not sure what it would buy us - we'd be optimizing
> for the case that essentially never occurs.
> 

I am not sure about the never occurs part of it, because we check
for the condition, so it could occur. I agree, it is a premature
optimization and could wait a little longer before going in.

>>
>>
>>
>> > +int container_task_count(const struct container *cont) {
>> > +     int count = 0;
>> > +     struct task_struct *g, *p;
>> > +     struct container_subsys_state *css;
>> > +     int subsys_id;
>> > +     get_first_subsys(cont, &css, &subsys_id);
>> > +
>> > +     read_lock(&tasklist_lock);
>>
>> Can be replaced with rcu_read_lock() and rcu_read_unlock()
> 
> Are you sure about that? I see many users of
> do_each_thread()/while_each_thread() taking a lock on tasklist_lock,
> and only one (fs/binfmt_elf.c) that's clearly relying on an RCU
> critical sections. Documentation?
> 

I suspect they are all pending conversions to be made.
Eric is the expert on this. Meanwhile here's a couple of
pointers. Quoting from the second URL

"We don't need the tasklist_lock to safely iterate through processes
anymore."

http://www.linuxjournal.com/article/6993 (please see incremental use
of RCU) and
http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.17/2.6.17-mm2/broken-out/proc-remove-tasklist_lock-from-proc_pid_readdir.patch

>>
>> Any chance we could get a per-container task list? It will
>> help subsystem writers as well.
> 
> It would be possible, yes - but we probably wouldn't want the overhead
> (additional ref counts and list manipulations on every fork/exit) of
> it on by default. We could make it a config option that particular
> subsystems could select.
> 
> I guess the question is how useful is this really, compared to just
> doing a do_each_thread() and seeing which tasks are in the container?
> Certainly that's a non-trivial operation, but in what circumstances is
> it really necessary to do it?
> 
> Paul


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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