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SubjectRE: [PATCHv4] procfs: show hierarchy of pid namespace
Date


> -----Original Message-----
> From: Oleg Nesterov [mailto:oleg@redhat.com]
> Sent: Friday, October 10, 2014 5:34 AM
> To: Chen, Hanxiao/陈 晗霄
> Cc: containers@lists.linux-foundation.org; linux-kernel@vger.kernel.org; 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@redhat.com]
> > >
> > > 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

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