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SubjectRe: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2
On Thu, Dec 08, 2011 at 05:35:35PM +0100, Oleg Nesterov wrote:
...
>
> However, ->children list is not rcu-safe, this means that even
> list_for_each() itself is not safe. Either you need tasklist or
> we can probably make it rcu-safe...
>

Andrew, Oleg, does the below one look more less fine? Note the
tasklist_lock is back and it worries me a bit since I imagine
one could be endlessly reading some /proc/<pid>/children file
increasing contention over this lock on the whole system
(regardless the fact that it's take for read only).

Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.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.

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,
a nit pointed by Tejun
- Documentation update
- tasklist_lock is back, Oleg pointed that ->children list
is actually not rcu-safe

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
---
Documentation/filesystems/proc.txt | 11 +++
fs/proc/array.c | 127 +++++++++++++++++++++++++++++++++++++
fs/proc/base.c | 1
fs/proc/internal.h | 6 +
4 files changed, 145 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 +1546,13 @@ 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 a 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 here.
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 +547,130 @@ int proc_pid_statm(struct seq_file *m, s

return 0;
}
+
+static struct list_head *
+children_get_at(struct proc_pid_children_iter *iter, loff_t pos)
+{
+ struct task_struct *t = iter->leader;
+ do {
+ struct list_head *p;
+ list_for_each(p, &t->children) {
+ if (pos-- == 0) {
+ iter->last = t;
+ return p;
+ }
+ }
+ } while_each_thread(iter->leader, t);
+
+ /* To make sure it's never in unknown state */
+ iter->last = NULL;
+
+ return NULL;
+}
+
+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 proc_pid_children_iter *iter = seq->private;
+
+ read_lock(&tasklist_lock);
+ return children_get_at(iter, *pos);
+}
+
+static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+ struct proc_pid_children_iter *iter = seq->private;
+ struct list_head *next = NULL;
+
+ if (iter->last) {
+ if (list_is_last(v, &iter->last->children)) {
+ while_each_thread(iter->leader, iter->last) {
+ if (!list_empty(&iter->last->children)) {
+ next = iter->last->children.next;
+ goto found;
+ }
+ }
+ iter->last = NULL;
+ } else
+ next = ((struct list_head *)v)->next;
+ }
+
+found:
+ ++*pos;
+ if (!next)
+ seq_printf(seq, "\n");
+ return next;
+}
+
+static void children_seq_stop(struct seq_file *seq, void *v)
+{
+ struct proc_pid_children_iter *iter = seq->private;
+
+ iter->last = NULL;
+ read_unlock(&tasklist_lock);
+}
+
+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 proc_pid_children_iter *iter = NULL;
+ struct task_struct *task = NULL;
+ int ret;
+
+ ret = -ENOMEM;
+ iter = kmalloc(sizeof(*iter), GFP_KERNEL);
+ if (!iter)
+ goto err;
+
+ ret = -ENOENT;
+ task = get_proc_task(inode);
+ if (!task)
+ goto err;
+
+ ret = seq_open(file, &children_seq_ops);
+ if (!ret) {
+ struct seq_file *m = file->private_data;
+ m->private = iter;
+ iter->leader = task;
+ iter->last = task;
+ }
+
+err:
+ if (ret) {
+ if (task)
+ put_task_struct(task);
+ kfree(iter);
+ }
+
+ return ret;
+}
+
+int children_seq_release(struct inode *inode, struct file *file)
+{
+ struct seq_file *m = file->private_data;
+ struct proc_pid_children_iter *iter = m->private;
+
+ put_task_struct(iter->leader);
+ kfree(iter);
+ 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 +53,12 @@ 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);

+struct proc_pid_children_iter {
+ struct task_struct *leader;
+ struct task_struct *last;
+};
+
+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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