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Subject[patch 1/4] 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>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
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>
---
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
@@ -3384,6 +3384,7 @@ 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;


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