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Message-ID: <485C9DA0.9080803@cn.fujitsu.com>
Date: Sat, 21 Jun 2008 14:20:16 +0800
From: Li Zefan <lizf@...fujitsu.com>
To: Paul Menage <menage@...gle.com>
CC: LKML <linux-kernel@...r.kernel.org>,
Linux Containers <containers@...ts.linux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Paul Jackson <pj@....com>
Subject: Re: [RFC] [PATCH] cgroup: add "procs" control file
Paul Menage wrote:
> On Wed, Jun 18, 2008 at 1:02 AM, Li Zefan <lizf@...fujitsu.com> wrote:
>> - What to do if the attaching of a thread failed? continue to attach
>> other threads, or stop and return error?
>
> I think this is something that will have to be handled in the design
> of transactional cgroup attach.
>
Is the following proposal feasable?
- call can_attach() for each thread before attaching them into the new group.
This works for cpuset, doesn't it?
- the above may not always reasonable, for example for Kosaki-san's task cgroup.
in this case, we require the subsystem to provide a can_attach_thread_group(),
like:
static int task_cgroup_can_attach_group(struct cgroup_subsys *ss,
struct cgroup *cgrp, struct task_struct *tsk)
{
struct task_cgroup *taskcg = task_cgroup_from_cgrp(cgrp);
struct task_struct *t;
int ret = 0;
int nr_threads = 1;
for (t = next_thread(tsk); t != tsk; t = next_thread(t)
nr_threads++;
spin_lock(&taskcg->lock);
if (taskcg->nr_tasks + nr_threads > taskcg->max_tasks)
ret = -EBUSY;
spin_unlock(&taskcg->lock);
return ret;
}
>> - When a sub-thread of a process is in the cgroup, but not its thread
>> cgroup leader, what to do when 'cat procs'? just skip those threads?
>
> Sounds reasonable. I think that in general the procs file is more
> useful as a write API than a read API anyway, for the reasons you
> indicate there.
>
>
>> + tsk = attach_get_task(cgrp, pidbuf);
>> + if (IS_ERR(tsk))
>> + return PTR_ERR(tsk);
>> +
>> + /* attach thread group leader */
>
> Should we check that this is in fact a thread group leader?
>
No need actually, I added this check originally and then removed it, but
forgot to remove the comment.
>> +
>> + /* attach all sub-threads */
>> + rcu_read_lock();
>
> cgroup_attach_task() calls synchronize_rcu(), so it doesn't seem
> likely that rcu_read_lock() is useful here, and might even deadlock?
>
> What are you trying to protect against with the RCU lock?
>
Ah yes, bad here. I am trying to protect the thread list.
>> {
>> + .name = "procs",
>
> Maybe call it "cgroup.procs" to avoid name clashes in future? We had a
> debate a while back where I tried to get the cgroup files like "tasks"
> and "notify_on_release" prefixed with "cgroup." , which were argued
> against on grounds of backwards compatibility. But there's no
> compatibility issue here. The only question is whether it's too ugly
> to have the legacy filenames without a prefix and the new ones with a
> prefix.
>
Yes it's ugly.. Is possible name clash of "procs" a kind of breaking
compatibility that should be avoid in any case?
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