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Message-ID: <20111207215228.GU21678@moon>
Date: Thu, 8 Dec 2011 01:52:28 +0400
From: Cyrill Gorcunov <gorcunov@...il.com>
To: Oleg Nesterov <oleg@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Tejun Heo <tj@...nel.org>
Cc: Vasiliy Kulikov <segoon@...nwall.com>,
Pavel Emelyanov <xemul@...allels.com>,
Serge Hallyn <serge.hallyn@...onical.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Andrew Vagin <avagin@...nvz.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2
On Thu, Dec 08, 2011 at 12:34:50AM +0400, Cyrill Gorcunov wrote:
...
>
> So while adding new task to a list is not a problem (the reader
> will simply not notice it) removing task from a list is a bit
> tricky, but as far as I see switch_task_namespaces (from exit_notify)
> uses synchronize_rcu as well as release_task calls for call_rcu
> for put_task_struct) so I think we will not get any wrong dereference
> in
>
> static int children_seq_show(struct seq_file *seq, void *v)
> {
> struct task_struct *task = container_of(v, struct task_struct, sibling);
> return seq_printf(seq, " %lu", (unsigned long)pid_vnr(task_pid(task)));
> }
>
> while we're in rcu reader section. I might be wrong of course, so
> please verify this claim.
>
An updated version is below (I've updated changelog and docs as well).
Please review if you get a chance. Complains are welcome of course.
Cyrill
---
From: Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v4
There is no easy way to make a reverse parent->children chain
from arbitrary <pid> (while parent pid is provided in "PPid"
field of /proc/<pid>/status).
So instead of walking over all pids in the system to figure out which
children a task have -- we add explicit /proc/<pid>/children entry,
because kernel already has this kind of information but it is not
yet exported. This is a first level children, not the whole process
tree, neither the process threads are identified with this interface.
Note that filling user-space buffer is done in iterative manner
where each iteration is protected via RCU read lock, which means
if between iterations a new task is added it might be missed in
output and to have a consistent image of which children are really
present in system the task should be either frozen or stopped
together with all children.
v2:
- Kame suggested to use a separated /proc/<pid>/children entry
instead of poking /proc/<pid>/status
- Andew suggested to use rcu facility instead of locking
tasklist_lock
- Tejun pointed that non-seekable seq file might not be
enough for tasks with large number of children
v3:
- To be on a safe side use %lu format for pid_t printing
v4:
- New line get printed when sequence ends not at seq->stop,
nit pointed by Tejun
- documentation update
Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Pavel Emelyanov <xemul@...allels.com>
Cc: Serge Hallyn <serge.hallyn@...onical.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Pavel Emelyanov <xemul@...allels.com>
---
Documentation/filesystems/proc.txt | 19 +++++++++
fs/proc/array.c | 72 +++++++++++++++++++++++++++++++++++++
fs/proc/base.c | 1
fs/proc/internal.h | 1
4 files changed, 93 insertions(+)
Index: linux-2.6.git/Documentation/filesystems/proc.txt
===================================================================
--- linux-2.6.git.orig/Documentation/filesystems/proc.txt
+++ linux-2.6.git/Documentation/filesystems/proc.txt
@@ -40,6 +40,7 @@ Table of Contents
3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
3.5 /proc/<pid>/mountinfo - Information about mounts
3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
+ 3.7 /proc/<pid>/children - Information about task children
------------------------------------------------------------------------------
@@ -1545,3 +1546,21 @@ a task to set its own or one of its thre
is limited in size compared to the cmdline value, so writing anything longer
then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
comm value.
+
+3.7 /proc/<pid>/children - Information about task children
+--------------------------------------------------------------
+This file provides a fast way to retrieve first level children pids
+of the task pointed by <pid>. The format is a stream of pids separated
+by space with a new line at the end. If a task has no children at
+all -- only a new line returned. Note the "first level" here -- if
+a task child has own children they will not be printed there, one
+need to read /proc/<children-pid>/children to obtain such pids.
+The same applies to threads -- they are not counted via this interface,
+one need to read /proc/<pid>/status to find all threads the task created.
+
+Note that filling user-space buffer is done in iterative manner where
+each iteration is protected via RCU read lock, which means if between
+iterations a new task is added it might be missed in output and
+to have a consistent image of which children are really present in
+system the task should be either frozen or stopped together with
+all children.
Index: linux-2.6.git/fs/proc/array.c
===================================================================
--- linux-2.6.git.orig/fs/proc/array.c
+++ linux-2.6.git/fs/proc/array.c
@@ -547,3 +547,75 @@ int proc_pid_statm(struct seq_file *m, s
return 0;
}
+
+static int children_seq_show(struct seq_file *seq, void *v)
+{
+ struct task_struct *task = container_of(v, struct task_struct, sibling);
+ return seq_printf(seq, " %lu", (unsigned long)pid_vnr(task_pid(task)));
+}
+
+static void *children_seq_start(struct seq_file *seq, loff_t *pos)
+{
+ struct task_struct *task = seq->private;
+
+ rcu_read_lock();
+ return seq_list_start(&task->children, *pos);
+}
+
+static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+ struct task_struct *task = seq->private;
+ struct list_head *head;
+
+ head = seq_list_next(v, &task->children, pos);
+ if (!head)
+ seq_printf(seq, "\n");
+ return head;
+}
+
+static void children_seq_stop(struct seq_file *seq, void *v)
+{
+ rcu_read_unlock();
+}
+
+static const struct seq_operations children_seq_ops = {
+ .start = children_seq_start,
+ .next = children_seq_next,
+ .stop = children_seq_stop,
+ .show = children_seq_show,
+};
+
+static int children_seq_open(struct inode *inode, struct file *file)
+{
+ struct task_struct *task;
+ int ret;
+
+ task = get_proc_task(inode);
+ if (!task)
+ return -ENOENT;
+
+ ret = seq_open(file, &children_seq_ops);
+ if (!ret) {
+ struct seq_file *m = file->private_data;
+ m->private = task;
+ } else
+ put_task_struct(task);
+
+ return ret;
+}
+int children_seq_release(struct inode *inode, struct file *file)
+{
+ struct seq_file *m = file->private_data;
+ if (m->private)
+ put_task_struct(m->private);
+
+ seq_release(inode, file);
+ return 0;
+}
+
+const struct file_operations proc_pid_children_operations = {
+ .open = children_seq_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = children_seq_release,
+};
Index: linux-2.6.git/fs/proc/base.c
===================================================================
--- linux-2.6.git.orig/fs/proc/base.c
+++ linux-2.6.git/fs/proc/base.c
@@ -3204,6 +3204,7 @@ static const struct pid_entry tgid_base_
INF("cmdline", S_IRUGO, proc_pid_cmdline),
ONE("stat", S_IRUGO, proc_tgid_stat),
ONE("statm", S_IRUGO, proc_pid_statm),
+ REG("children", S_IRUGO, proc_pid_children_operations),
REG("maps", S_IRUGO, proc_maps_operations),
#ifdef CONFIG_NUMA
REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
Index: linux-2.6.git/fs/proc/internal.h
===================================================================
--- linux-2.6.git.orig/fs/proc/internal.h
+++ linux-2.6.git/fs/proc/internal.h
@@ -53,6 +53,7 @@ extern int proc_pid_statm(struct seq_fil
struct pid *pid, struct task_struct *task);
extern loff_t mem_lseek(struct file *file, loff_t offset, int orig);
+extern const struct file_operations proc_pid_children_operations;
extern const struct file_operations proc_maps_operations;
extern const struct file_operations proc_numa_maps_operations;
extern const struct file_operations proc_smaps_operations;
--
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