[PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Cyrill Gorcunov]

If it's needed I can wrap all this with CONFIG_CHECKPOINT_RESTORE, should I?

---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

When we do checkpoint of a task we need to know the list of children
the task, has but there is no easy and fast way to generate reverse
parent->children chain from arbitrary <pid> (while a parent pid is
provided in "PPid" field of /proc/<pid>/status).

So instead of walking over all pids in the system (creating one big process
tree in memory, just to figure out which children a task has) -- we add
explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
this kind of information but it is not yet exported.

This is a first level children, not the whole process tree.

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

v5:
 - Oleg suggested to make /proc/<pid>/task/<tid>/children
   instead of global /proc/<pid>/children, which eliminates
   hardness related to threads and children migration, and
   allows patch to be a way simplier.

v6:
 - Drop ptrace_may_access tests, pids are can be found anyway
   so nothing to protect here.
 - Update comments and docs, pointed by Oleg.

v7:
 - Use get_pid over proc-pid directly, to simplify
   code, pointed by Oleg.

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: Oleg Nesterov <oleg@redhat.com>
---
 Documentation/filesystems/proc.txt |   18 ++++
 fs/proc/array.c                    |  147 +++++++++++++++++++++++++++++++++++++
 fs/proc/base.c                     |    1 
 fs/proc/internal.h                 |    1 
 4 files changed, 167 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>/task/<tid>/children - Information about task children
 
   4	Configuring procfs
   4.1	Mount options
@@ -1549,6 +1550,23 @@ then the kernel's TASK_COMM_LEN (current
 comm value.
 
 
+3.7	/proc/<pid>/task/<tid>/children - Information about task children
+-------------------------------------------------------------------------
+This file provides a fast way to retrieve first level children pids
+of a task pointed by <pid>/<tid> pair. The format is a space separated
+stream of pids.
+
+Note the "first level" here -- if a child has own children they will
+not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
+to obtain the descendants.
+
+Since this interface is intended to be fast and cheap it doesn't
+guarantee to provide precise results and some children might be
+skipped, especially if they've exited right after we printed their
+pids, so one need to either stop or freeze processes being inspected
+if precise results are needed.
+
+
 ------------------------------------------------------------------------------
 Configuring procfs
 ------------------------------------------------------------------------------
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 +547,150 @@ int proc_pid_statm(struct seq_file *m, s
 
 	return 0;
 }
+
+struct proc_pid_children_iter {
+	struct pid_namespace *pid_ns;
+	struct pid *parent_pid;
+};
+
+static struct pid *
+get_children_pid(struct proc_pid_children_iter *iter, struct pid *pid_prev, loff_t pos)
+{
+	struct task_struct *start, *task;
+	struct pid *pid = NULL;
+
+	read_lock(&tasklist_lock);
+
+	start = pid_task(iter->parent_pid, PIDTYPE_PID);
+	if (!start)
+		goto out;
+
+	/*
+	 * Lets try to continue searching first, this gives
+	 * us significant speedup on children-rich processes.
+	 */
+	if (pid_prev) {
+		task = pid_task(pid_prev, PIDTYPE_PID);
+		if (task && task->real_parent == start &&
+		    !(list_empty(&task->sibling))) {
+			if (list_is_last(&task->sibling, &start->children))
+				goto out;
+			task = list_first_entry(&task->sibling,
+						struct task_struct, sibling);
+			pid = get_pid(task_pid(task));
+			goto out;
+		}
+	}
+
+	/*
+	 * Slow search case.
+	 *
+	 * We might miss some children here if children
+	 * are exited while we were not holding the lock,
+	 * but it was never promised to be accurate that
+	 * much.
+	 *
+	 * "Just suppose that the parent sleeps, but N children
+	 *  exit after we printed their tids. Now the slow paths
+	 *  skips N extra children, we miss N tasks." (c)
+	 *
+	 * So one need to stop or freeze the leader and all
+	 * its children to get a precise result.
+	 */
+	list_for_each_entry(task, &start->children, sibling) {
+		if (pos-- == 0) {
+			pid = get_pid(task_pid(task));
+			break;
+		}
+	}
+
+out:
+	read_unlock(&tasklist_lock);
+	return pid;
+}
+
+static int children_seq_show(struct seq_file *seq, void *v)
+{
+	struct proc_pid_children_iter *iter = seq->private;
+	unsigned long pid = (unsigned long)pid_nr_ns(v, iter->pid_ns);
+
+	return seq_printf(seq, " %lu", pid);
+}
+
+static void *children_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	return get_children_pid(seq->private, NULL, *pos);
+}
+
+static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct proc_pid_children_iter *iter = seq->private;
+	struct pid *pid = NULL;
+
+	pid = get_children_pid(iter, v, *pos + 1);
+	if (!pid)
+		seq_printf(seq, "\n");
+	put_pid(v);
+
+	++*pos;
+	return pid;
+}
+
+static void children_seq_stop(struct seq_file *seq, void *v)
+{
+	put_pid(v);
+}
+
+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 = 0;
+
+	iter = kmalloc(sizeof(*iter), GFP_KERNEL);
+	if (!iter) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	ret = seq_open(file, &children_seq_ops);
+	if (!ret) {
+		struct seq_file *m = file->private_data;
+		m->private = iter;
+
+		iter->parent_pid = get_pid(proc_pid(inode));
+		iter->pid_ns = inode->i_sb->s_fs_info;
+	}
+
+err:
+	if (ret)
+		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_pid(iter->parent_pid);
+	kfree(iter);
+
+	seq_release(inode, file);
+	return 0;
+}
+
+const struct file_operations proc_tid_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
@@ -3454,6 +3454,7 @@ static const struct pid_entry tid_base_s
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_maps_operations),
+	REG("children",  S_IRUGO, proc_tid_children_operations),
 #ifdef CONFIG_NUMA
 	REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
 #endif
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_tid_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;

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Cyrill Gorcunov]

On Thu, Jan 19, 2012 at 07:51:12PM +0400, Cyrill Gorcunov wrote:
> If it's needed I can wrap all this with CONFIG_CHECKPOINT_RESTORE, should I?
> 
> ---

Oleg, if only I'm not missing something obvious you meant handling like below?

	Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8

When we do checkpoint of a task we need to know the list of children
the task, has but there is no easy and fast way to generate reverse
parent->children chain from arbitrary <pid> (while a parent pid is
provided in "PPid" field of /proc/<pid>/status).

So instead of walking over all pids in the system (creating one big process
tree in memory, just to figure out which children a task has) -- we add
explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
this kind of information but it is not yet exported.

This is a first level children, not the whole process tree.

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

v5:
 - Oleg suggested to make /proc/<pid>/task/<tid>/children
   instead of global /proc/<pid>/children, which eliminates
   hardness related to threads and children migration, and
   allows patch to be a way simplier.

v6:
 - Drop ptrace_may_access tests, pids are can be found anyway
   so nothing to protect here.
 - Update comments and docs, pointed by Oleg.

v7:
 - Use get_pid over proc-pid directly, to simplify
   code, pointed by Oleg.

v8:
 - Obtain a starting pid from the proc's inode directly.

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: Oleg Nesterov <oleg@redhat.com>
---
 Documentation/filesystems/proc.txt |   18 +++++
 fs/proc/array.c                    |  128 +++++++++++++++++++++++++++++++++++++
 fs/proc/base.c                     |    1 
 fs/proc/internal.h                 |    1 
 4 files changed, 148 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>/task/<tid>/children - Information about task children
 
   4	Configuring procfs
   4.1	Mount options
@@ -1549,6 +1550,23 @@ then the kernel's TASK_COMM_LEN (current
 comm value.
 
 
+3.7	/proc/<pid>/task/<tid>/children - Information about task children
+-------------------------------------------------------------------------
+This file provides a fast way to retrieve first level children pids
+of a task pointed by <pid>/<tid> pair. The format is a space separated
+stream of pids.
+
+Note the "first level" here -- if a child has own children they will
+not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
+to obtain the descendants.
+
+Since this interface is intended to be fast and cheap it doesn't
+guarantee to provide precise results and some children might be
+skipped, especially if they've exited right after we printed their
+pids, so one need to either stop or freeze processes being inspected
+if precise results are needed.
+
+
 ------------------------------------------------------------------------------
 Configuring procfs
 ------------------------------------------------------------------------------
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 +547,131 @@ int proc_pid_statm(struct seq_file *m, s
 
 	return 0;
 }
+
+struct proc_pid_children_iter {
+	struct pid_namespace *pid_ns;
+	struct pid *parent_pid;
+};
+
+static struct pid *
+get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
+{
+	struct task_struct *start, *task;
+	struct pid *pid = NULL;
+
+	read_lock(&tasklist_lock);
+
+	start = pid_task(proc_pid(inode), PIDTYPE_PID);
+	if (!start)
+		goto out;
+
+	/*
+	 * Lets try to continue searching first, this gives
+	 * us significant speedup on children-rich processes.
+	 */
+	if (pid_prev) {
+		task = pid_task(pid_prev, PIDTYPE_PID);
+		if (task && task->real_parent == start &&
+		    !(list_empty(&task->sibling))) {
+			if (list_is_last(&task->sibling, &start->children))
+				goto out;
+			task = list_first_entry(&task->sibling,
+						struct task_struct, sibling);
+			pid = get_pid(task_pid(task));
+			goto out;
+		}
+	}
+
+	/*
+	 * Slow search case.
+	 *
+	 * We might miss some children here if children
+	 * are exited while we were not holding the lock,
+	 * but it was never promised to be accurate that
+	 * much.
+	 *
+	 * "Just suppose that the parent sleeps, but N children
+	 *  exit after we printed their tids. Now the slow paths
+	 *  skips N extra children, we miss N tasks." (c)
+	 *
+	 * So one need to stop or freeze the leader and all
+	 * its children to get a precise result.
+	 */
+	list_for_each_entry(task, &start->children, sibling) {
+		if (pos-- == 0) {
+			pid = get_pid(task_pid(task));
+			break;
+		}
+	}
+
+out:
+	read_unlock(&tasklist_lock);
+	return pid;
+}
+
+static int children_seq_show(struct seq_file *seq, void *v)
+{
+	struct inode *inode = seq->private;
+	unsigned long pid;
+
+	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
+	return seq_printf(seq, " %lu", pid);
+}
+
+static void *children_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	return get_children_pid(seq->private, NULL, *pos);
+}
+
+static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct pid *pid = NULL;
+
+	pid = get_children_pid(seq->private, v, *pos + 1);
+	if (!pid)
+		seq_printf(seq, "\n");
+	put_pid(v);
+
+	++*pos;
+	return pid;
+}
+
+static void children_seq_stop(struct seq_file *seq, void *v)
+{
+	put_pid(v);
+}
+
+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 seq_file *m;
+	int ret;
+
+	ret = seq_open(file, &children_seq_ops);
+	if (ret)
+		return ret;
+
+	m = file->private_data;
+	m->private = inode;
+
+	return ret;
+}
+
+int children_seq_release(struct inode *inode, struct file *file)
+{
+	seq_release(inode, file);
+	return 0;
+}
+
+const struct file_operations proc_tid_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
@@ -3454,6 +3454,7 @@ static const struct pid_entry tid_base_s
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_maps_operations),
+	REG("children",  S_IRUGO, proc_tid_children_operations),
 #ifdef CONFIG_NUMA
 	REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
 #endif
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_tid_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;

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Oleg Nesterov]

On 01/19, Cyrill Gorcunov wrote:
>
> On Thu, Jan 19, 2012 at 07:51:12PM +0400, Cyrill Gorcunov wrote:
> > If it's needed I can wrap all this with CONFIG_CHECKPOINT_RESTORE, should I?
> >
> > ---
>
> Oleg, if only I'm not missing something obvious you meant handling like below?

Yes, but...

> +struct proc_pid_children_iter {
> +	struct pid_namespace *pid_ns;
> +	struct pid *parent_pid;
> +};

you forgot to remove this definition.

> +static int children_seq_show(struct seq_file *seq, void *v)
> +{
> +	struct inode *inode = seq->private;
> +	unsigned long pid;
> +
> +	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
> +	return seq_printf(seq, " %lu", pid);
> +}

just noticed... why unsigned long and %lu? afaics pid_t/%d should work
without any typecasts.

Oleg.

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Cyrill Gorcunov]

On Thu, Jan 19, 2012 at 06:57:00PM +0100, Oleg Nesterov wrote:
> On 01/19, Cyrill Gorcunov wrote:
> >
> > On Thu, Jan 19, 2012 at 07:51:12PM +0400, Cyrill Gorcunov wrote:
> > > If it's needed I can wrap all this with CONFIG_CHECKPOINT_RESTORE, should I?
> > >
> > > ---
> >
> > Oleg, if only I'm not missing something obvious you meant handling like below?
> 
> Yes, but...
> 
> > +struct proc_pid_children_iter {
> > +	struct pid_namespace *pid_ns;
> > +	struct pid *parent_pid;
> > +};
> 
> you forgot to remove this definition.
> 

No, I rather forgot to quilt refresh :)

> > +static int children_seq_show(struct seq_file *seq, void *v)
> > +{
> > +	struct inode *inode = seq->private;
> > +	unsigned long pid;
> > +
> > +	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
> > +	return seq_printf(seq, " %lu", pid);
> > +}
> 
> just noticed... why unsigned long and %lu? afaics pid_t/%d should work
> without any typecasts.
> 

I'm not sure how important it is, but Andrew mentioned in one of email
that we might be moving from pid_t from int to long one day (which of
course will require extreme huge work on checking code where int->long
transition might cause problems). So I thought why should I wait then?

[ Andrew, am I correct? ]

But since I'll be refreshig patch anyway, I drop this.

	Cyrill

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Cyrill Gorcunov]

On Thu, Jan 19, 2012 at 10:09:51PM +0400, Cyrill Gorcunov wrote:
> No, I rather forgot to quilt refresh :)
> 
> > > +static int children_seq_show(struct seq_file *seq, void *v)
> > > +{
> > > +	struct inode *inode = seq->private;
> > > +	unsigned long pid;
> > > +
> > > +	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
> > > +	return seq_printf(seq, " %lu", pid);
> > > +}
> > 
> > just noticed... why unsigned long and %lu? afaics pid_t/%d should work
> > without any typecasts.
> > 
> 
> I'm not sure how important it is, but Andrew mentioned in one of email
> that we might be moving from pid_t from int to long one day (which of
> course will require extreme huge work on checking code where int->long
> transition might cause problems). So I thought why should I wait then?
> 
> [ Andrew, am I correct? ]
> 
> But since I'll be refreshig patch anyway, I drop this.
> 

Here is refreshed one for sure.

	Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8

When we do checkpoint of a task we need to know the list of children
the task, has but there is no easy and fast way to generate reverse
parent->children chain from arbitrary <pid> (while a parent pid is
provided in "PPid" field of /proc/<pid>/status).

So instead of walking over all pids in the system (creating one big process
tree in memory, just to figure out which children a task has) -- we add
explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
this kind of information but it is not yet exported.

This is a first level children, not the whole process tree.

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

v5:
 - Oleg suggested to make /proc/<pid>/task/<tid>/children
   instead of global /proc/<pid>/children, which eliminates
   hardness related to threads and children migration, and
   allows patch to be a way simplier.

v6:
 - Drop ptrace_may_access tests, pids are can be found anyway
   so nothing to protect here.
 - Update comments and docs, pointed by Oleg.

v7:
 - Use get_pid over proc-pid directly, to simplify
   code, pointed by Oleg.

v8:
 - Obtain a starting pid from the proc's inode directly.

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: Oleg Nesterov <oleg@redhat.com>
---
 Documentation/filesystems/proc.txt |   18 +++++
 fs/proc/array.c                    |  123 +++++++++++++++++++++++++++++++++++++
 fs/proc/base.c                     |    1 
 fs/proc/internal.h                 |    1 
 4 files changed, 143 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>/task/<tid>/children - Information about task children
 
   4	Configuring procfs
   4.1	Mount options
@@ -1549,6 +1550,23 @@ then the kernel's TASK_COMM_LEN (current
 comm value.
 
 
+3.7	/proc/<pid>/task/<tid>/children - Information about task children
+-------------------------------------------------------------------------
+This file provides a fast way to retrieve first level children pids
+of a task pointed by <pid>/<tid> pair. The format is a space separated
+stream of pids.
+
+Note the "first level" here -- if a child has own children they will
+not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
+to obtain the descendants.
+
+Since this interface is intended to be fast and cheap it doesn't
+guarantee to provide precise results and some children might be
+skipped, especially if they've exited right after we printed their
+pids, so one need to either stop or freeze processes being inspected
+if precise results are needed.
+
+
 ------------------------------------------------------------------------------
 Configuring procfs
 ------------------------------------------------------------------------------
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 +547,126 @@ int proc_pid_statm(struct seq_file *m, s
 
 	return 0;
 }
+
+static struct pid *
+get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
+{
+	struct task_struct *start, *task;
+	struct pid *pid = NULL;
+
+	read_lock(&tasklist_lock);
+
+	start = pid_task(proc_pid(inode), PIDTYPE_PID);
+	if (!start)
+		goto out;
+
+	/*
+	 * Lets try to continue searching first, this gives
+	 * us significant speedup on children-rich processes.
+	 */
+	if (pid_prev) {
+		task = pid_task(pid_prev, PIDTYPE_PID);
+		if (task && task->real_parent == start &&
+		    !(list_empty(&task->sibling))) {
+			if (list_is_last(&task->sibling, &start->children))
+				goto out;
+			task = list_first_entry(&task->sibling,
+						struct task_struct, sibling);
+			pid = get_pid(task_pid(task));
+			goto out;
+		}
+	}
+
+	/*
+	 * Slow search case.
+	 *
+	 * We might miss some children here if children
+	 * are exited while we were not holding the lock,
+	 * but it was never promised to be accurate that
+	 * much.
+	 *
+	 * "Just suppose that the parent sleeps, but N children
+	 *  exit after we printed their tids. Now the slow paths
+	 *  skips N extra children, we miss N tasks." (c)
+	 *
+	 * So one need to stop or freeze the leader and all
+	 * its children to get a precise result.
+	 */
+	list_for_each_entry(task, &start->children, sibling) {
+		if (pos-- == 0) {
+			pid = get_pid(task_pid(task));
+			break;
+		}
+	}
+
+out:
+	read_unlock(&tasklist_lock);
+	return pid;
+}
+
+static int children_seq_show(struct seq_file *seq, void *v)
+{
+	struct inode *inode = seq->private;
+	pid_t pid;
+
+	pid = pid_nr_ns(v, inode->i_sb->s_fs_info);
+	return seq_printf(seq, " %d", pid);
+}
+
+static void *children_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	return get_children_pid(seq->private, NULL, *pos);
+}
+
+static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct pid *pid = NULL;
+
+	pid = get_children_pid(seq->private, v, *pos + 1);
+	if (!pid)
+		seq_printf(seq, "\n");
+	put_pid(v);
+
+	++*pos;
+	return pid;
+}
+
+static void children_seq_stop(struct seq_file *seq, void *v)
+{
+	put_pid(v);
+}
+
+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 seq_file *m;
+	int ret;
+
+	ret = seq_open(file, &children_seq_ops);
+	if (ret)
+		return ret;
+
+	m = file->private_data;
+	m->private = inode;
+
+	return ret;
+}
+
+int children_seq_release(struct inode *inode, struct file *file)
+{
+	seq_release(inode, file);
+	return 0;
+}
+
+const struct file_operations proc_tid_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
@@ -3454,6 +3454,7 @@ static const struct pid_entry tid_base_s
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
 	REG("maps",      S_IRUGO, proc_maps_operations),
+	REG("children",  S_IRUGO, proc_tid_children_operations),
 #ifdef CONFIG_NUMA
 	REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
 #endif
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_tid_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;

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Eric W. Biederman]

Cyrill Gorcunov <gorcunov@gmail.com> writes:

> On Thu, Jan 19, 2012 at 06:57:00PM +0100, Oleg Nesterov wrote:
>> On 01/19, Cyrill Gorcunov wrote:
>> >
>> > On Thu, Jan 19, 2012 at 07:51:12PM +0400, Cyrill Gorcunov wrote:
>> > > If it's needed I can wrap all this with CONFIG_CHECKPOINT_RESTORE, should I?
>> > >
>> > > ---
>> >
>> > Oleg, if only I'm not missing something obvious you meant handling like below?
>> 
>> Yes, but...
>> 
>> > +struct proc_pid_children_iter {
>> > +	struct pid_namespace *pid_ns;
>> > +	struct pid *parent_pid;
>> > +};
>> 
>> you forgot to remove this definition.
>> 
>
> No, I rather forgot to quilt refresh :)
>
>> > +static int children_seq_show(struct seq_file *seq, void *v)
>> > +{
>> > +	struct inode *inode = seq->private;
>> > +	unsigned long pid;
>> > +
>> > +	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
>> > +	return seq_printf(seq, " %lu", pid);
>> > +}
>> 
>> just noticed... why unsigned long and %lu? afaics pid_t/%d should work
>> without any typecasts.
>> 
>
> I'm not sure how important it is, but Andrew mentioned in one of email
> that we might be moving from pid_t from int to long one day (which of
> course will require extreme huge work on checking code where int->long
> transition might cause problems). So I thought why should I wait then?
>
> [ Andrew, am I correct? ]

It is going to take a lot to get to a > 32bit pid value.  I would not
worry about it today unless there is some 64bit arch somewhere that defines
pid_t as long.

Eric

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Cyrill Gorcunov]

On Thu, Jan 19, 2012 at 03:14:05PM -0800, Eric W. Biederman wrote:
...
> >
> >> > +static int children_seq_show(struct seq_file *seq, void *v)
> >> > +{
> >> > +	struct inode *inode = seq->private;
> >> > +	unsigned long pid;
> >> > +
> >> > +	pid = (unsigned long)pid_nr_ns(v, inode->i_sb->s_fs_info);
> >> > +	return seq_printf(seq, " %lu", pid);
> >> > +}
> >> 
> >> just noticed... why unsigned long and %lu? afaics pid_t/%d should work
> >> without any typecasts.
> >> 
> >
> > I'm not sure how important it is, but Andrew mentioned in one of email
> > that we might be moving from pid_t from int to long one day (which of
> > course will require extreme huge work on checking code where int->long
> > transition might cause problems). So I thought why should I wait then?
> >
> > [ Andrew, am I correct? ]
> 
> It is going to take a lot to get to a > 32bit pid value.  I would not
> worry about it today unless there is some 64bit arch somewhere that defines
> pid_t as long.
> 

Yeah, it seems I overdone here ;) I've sent updated version with pid_t.

	Cyrill

Re: [PATCH] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v7

[Oleg Nesterov]

On 01/19, Cyrill Gorcunov wrote:
>
> Here is refreshed one for sure.


Reviewed-by: Oleg Nesterov <oleg@redhat.com>


> From: Cyrill Gorcunov <gorcunov@openvz.org>
> Subject: fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8
> 
> When we do checkpoint of a task we need to know the list of children
> the task, has but there is no easy and fast way to generate reverse
> parent->children chain from arbitrary <pid> (while a parent pid is
> provided in "PPid" field of /proc/<pid>/status).
> 
> So instead of walking over all pids in the system (creating one big process
> tree in memory, just to figure out which children a task has) -- we add
> explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
> this kind of information but it is not yet exported.
> 
> This is a first level children, not the whole process tree.
> 
> 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
> 
> v5:
>  - Oleg suggested to make /proc/<pid>/task/<tid>/children
>    instead of global /proc/<pid>/children, which eliminates
>    hardness related to threads and children migration, and
>    allows patch to be a way simplier.
> 
> v6:
>  - Drop ptrace_may_access tests, pids are can be found anyway
>    so nothing to protect here.
>  - Update comments and docs, pointed by Oleg.
> 
> v7:
>  - Use get_pid over proc-pid directly, to simplify
>    code, pointed by Oleg.
> 
> v8:
>  - Obtain a starting pid from the proc's inode directly.
> 
> 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: Oleg Nesterov <oleg@redhat.com>
> ---
>  Documentation/filesystems/proc.txt |   18 +++++
>  fs/proc/array.c                    |  123 +++++++++++++++++++++++++++++++++++++
>  fs/proc/base.c                     |    1 
>  fs/proc/internal.h                 |    1 
>  4 files changed, 143 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>/task/<tid>/children - Information about task children
>  
>    4	Configuring procfs
>    4.1	Mount options
> @@ -1549,6 +1550,23 @@ then the kernel's TASK_COMM_LEN (current
>  comm value.
>  
>  
> +3.7	/proc/<pid>/task/<tid>/children - Information about task children
> +-------------------------------------------------------------------------
> +This file provides a fast way to retrieve first level children pids
> +of a task pointed by <pid>/<tid> pair. The format is a space separated
> +stream of pids.
> +
> +Note the "first level" here -- if a child has own children they will
> +not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
> +to obtain the descendants.
> +
> +Since this interface is intended to be fast and cheap it doesn't
> +guarantee to provide precise results and some children might be
> +skipped, especially if they've exited right after we printed their
> +pids, so one need to either stop or freeze processes being inspected
> +if precise results are needed.
> +
> +
>  ------------------------------------------------------------------------------
>  Configuring procfs
>  ------------------------------------------------------------------------------
> 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 +547,126 @@ int proc_pid_statm(struct seq_file *m, s
>  
>  	return 0;
>  }
> +
> +static struct pid *
> +get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
> +{
> +	struct task_struct *start, *task;
> +	struct pid *pid = NULL;
> +
> +	read_lock(&tasklist_lock);
> +
> +	start = pid_task(proc_pid(inode), PIDTYPE_PID);
> +	if (!start)
> +		goto out;
> +
> +	/*
> +	 * Lets try to continue searching first, this gives
> +	 * us significant speedup on children-rich processes.
> +	 */
> +	if (pid_prev) {
> +		task = pid_task(pid_prev, PIDTYPE_PID);
> +		if (task && task->real_parent == start &&
> +		    !(list_empty(&task->sibling))) {
> +			if (list_is_last(&task->sibling, &start->children))
> +				goto out;
> +			task = list_first_entry(&task->sibling,
> +						struct task_struct, sibling);
> +			pid = get_pid(task_pid(task));
> +			goto out;
> +		}
> +	}
> +
> +	/*
> +	 * Slow search case.
> +	 *
> +	 * We might miss some children here if children
> +	 * are exited while we were not holding the lock,
> +	 * but it was never promised to be accurate that
> +	 * much.
> +	 *
> +	 * "Just suppose that the parent sleeps, but N children
> +	 *  exit after we printed their tids. Now the slow paths
> +	 *  skips N extra children, we miss N tasks." (c)
> +	 *
> +	 * So one need to stop or freeze the leader and all
> +	 * its children to get a precise result.
> +	 */
> +	list_for_each_entry(task, &start->children, sibling) {
> +		if (pos-- == 0) {
> +			pid = get_pid(task_pid(task));
> +			break;
> +		}
> +	}
> +
> +out:
> +	read_unlock(&tasklist_lock);
> +	return pid;
> +}
> +
> +static int children_seq_show(struct seq_file *seq, void *v)
> +{
> +	struct inode *inode = seq->private;
> +	pid_t pid;
> +
> +	pid = pid_nr_ns(v, inode->i_sb->s_fs_info);
> +	return seq_printf(seq, " %d", pid);
> +}
> +
> +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +	return get_children_pid(seq->private, NULL, *pos);
> +}
> +
> +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +	struct pid *pid = NULL;
> +
> +	pid = get_children_pid(seq->private, v, *pos + 1);
> +	if (!pid)
> +		seq_printf(seq, "\n");
> +	put_pid(v);
> +
> +	++*pos;
> +	return pid;
> +}
> +
> +static void children_seq_stop(struct seq_file *seq, void *v)
> +{
> +	put_pid(v);
> +}
> +
> +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 seq_file *m;
> +	int ret;
> +
> +	ret = seq_open(file, &children_seq_ops);
> +	if (ret)
> +		return ret;
> +
> +	m = file->private_data;
> +	m->private = inode;
> +
> +	return ret;
> +}
> +
> +int children_seq_release(struct inode *inode, struct file *file)
> +{
> +	seq_release(inode, file);
> +	return 0;
> +}
> +
> +const struct file_operations proc_tid_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
> @@ -3454,6 +3454,7 @@ static const struct pid_entry tid_base_s
>  	ONE("stat",      S_IRUGO, proc_tid_stat),
>  	ONE("statm",     S_IRUGO, proc_pid_statm),
>  	REG("maps",      S_IRUGO, proc_maps_operations),
> +	REG("children",  S_IRUGO, proc_tid_children_operations),
>  #ifdef CONFIG_NUMA
>  	REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
>  #endif
> 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_tid_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;