RE: [PATCHv4] procfs: show hierarchy of pid namespace

[Chen, Hanxiao]

> -----Original Message-----
> From: Oleg Nesterov [mailto:oleg@xxxxxxxxxx]
> Sent: Friday, October 10, 2014 5:34 AM
> To: Chen, Hanxiao/陈 晗霄
> Cc: containers@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Serge
> Hallyn; Eric W. Biederman; David Howells; Richard Weinberger; Pavel Emelyanov;
> Vasiliy Kulikov; Mateusz Guzik
> Subject: Re: [PATCHv4] procfs: show hierarchy of pid namespace
> 
> On 10/09, Chen, Hanxiao wrote:
> >
> > > From: Oleg Nesterov [mailto:oleg@xxxxxxxxxx]
> > >
> > > Hmm. We only want the tasks from our namespace, yes? Perhaps find_ge_pid()
> > > makes more sense?
> >
> > Only tasks from our ns is valid.
> > But how could find_ge_pid() do that?
> >
> > nr = 1;
> > while (nr < PID_MAX_LIMIT) {
> > 	find_ge_pid(nr, curr_ns);
> > 	list_add();
> > 	nr++;
> > }
> 
> something like this, except list_add() should obviously depend on
> is_child_reaper() check.
> 
> This can be more optimal in sub-namespaces, you do not need to abuse
> the global process list.
> 
> And if you change this code to use get_pid/put_pid, then you do not
> need to hold rcu_read_lock() throughout, you only need it around
> find_ge_pid + get_pid.
> 
> At the same time, for_each_process() in the global namespace can be
> faster if there are a lot of sub-threads.
> 
> > Perhaps that's not a good way.
> 
> OK, I won't insist.
> 
> although it would be nice to know why do you think this is bad.
> 
I worried about it may slower in global namespace.
But it will provide a great convenient way when query pid hierarchy
when not in init_pid_ns.


> > > > +		pid = task_pid(p);
> > >
> > > Well, in theory you need barrier() here. Or perhaps we should add
> > > ACCESS_ONCE() into task_pid()...
> >
> > You mean modify task_pid as:
> > return ACCESS_ONCE(task->pids[PIDTYPE_PID].pid;);
> 
> Yes. But not now an not in this patch of course. I'd suggest to add
> barrier() just in case.
> 
We can get rid of task_pid when we use find_ge_pid.
> 
> > > And imho it would be better to declare pidns_list/pidns_tree locally
> > > in nslist_proc_show() and pass them to the callees.
> >
> > That's a good idea.
> > Will changed in the next version.
> 
> Good. And I forgot to mention, in this case you do not need pidns_list_lock
> at all afaics.

Thanks for your comments.
I'll post a new patch using find_ge_pid + get_pid 

Thanks,
- Chen