linux-kernel - Re: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status

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Date:	Fri, 02 Dec 2011 18:17:08 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Pedro Alves <pedro@...esourcery.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status

>>>>> Yes, just like the task/ dir gives you directories named with the
>>>>> processes's thread ids.  Opening /proc/PID/children/PID-CHILD1/ would get
>>>>> you the same as opening /proc/PID-CHILD1/.  Just like
>>>>> opening /proc/PID/task/PID-CHILD1/ gets you (almost) the same as opening
>>>>> /proc/PID-CHILD1/.
>>>>
>>>> You cannot make the dentry named /proc/<pid1>/children/<pid2> be a hardlink on
>>>> the /proc/<pid2>. Thus you have to make arbitrary amount of inodes to point to
>>>> a single task. This brings unnecessary complexity and memory usage (by dentries
>>>> and proc inodes).
>>>
>>> How is this different from the _already existing_ /proc/<pid1>/task/ directory?
>>
>> Those living in /proc/<pid1>/task do not live in /proc. At all. This explains
>> everything below.
> 
> Well, except they do, and it doesn't.  The're not visible when
> listing /proc/, but they're there.  Try it.
> 
> $ls /proc/ | grep 854
> 
> (empty)
> 
> $ ls /proc/8167/task/854
> attr    clear_refs  cpuset   exe     io       loginuid  mountinfo  oom_adj        pagemap      sched      smaps  statm    wchan
> auxv    cmdline     cwd      fd      latency  maps      mounts     oom_score      personality  schedstat  stack  status
> cgroup  comm        environ  fdinfo  limits   mem       numa_maps  oom_score_adj  root         sessionid  stat   syscall
> 
> $ls /proc/854/
> attr       cgroup      comm             cwd      fd      latency   maps       mounts      numa_maps  oom_score_adj  root       sessionid  stat    syscall
> autogroup  clear_refs  coredump_filter  environ  fdinfo  limits    mem        mountstats  oom_adj    pagemap        sched      smaps      statm   task
> auxv       cmdline     cpuset           exe      io      loginuid  mountinfo  net         oom_score  personality    schedstat  stack      status  wchan
> 

O_O   OK, I was wrong, they do live there. But I consider this as bug.

Anyway -- my concern about unneeded memory overhead still stands.
Even a simple find /proc will result in smth like

/proc/1
/proc/1/children/2
/proc/1/children/2/children/4
/proc/1/children/3
/proc/1/children/3/children/5
/proc/2
/proc/2/children/4
/proc/3
/proc/3/children/5
/proc/4
/proc/5

Instead of

/proc/1
/proc/2
/proc/3
/proc/4
/proc/5

I.e. each task will be shown multiple times, which is not very fun, but memory exhaustive from my POV.

Thanks,
Pavel
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