[rfc] do not place sub-threads on task_struct->children list

[Oleg Nesterov <oleg@redhat.com>]

(change subject)

On 06/29, Oleg Nesterov wrote:
>
> On 06/25, Ratan Nalumasu wrote:
> >
> > I understood it in the _past_, but somehow forgotten the details--I have a
> > vague recollection that there was some special handling for the group leader
> > stuff.
> > However, to reconfirm, we believe that the
> > following condition in eligible_child() is good, right:
> > ===
> >        if ((wo->wo_flags & __WNOTHREAD) && wo->child_wait.private !=
> > p->parent)
> >                return 0;
> > ===
> >
> > I will run it on my test machines and see if everything looks good.
>
> OK, thanks.
>
> The only problem it uses ->parent, this conflicts with other out-of-tree
> ptrace changes...
>
> Roland, do you think we should do this change now or later?
>
> If now, then we can also do another nice optimization, and it is for free.
> See the untested patch below. It conflicts with those patches too, but
> in both cases fixups are trivial.

See the updated patch below, slightly tested.

Changes: s/reparent_thread/reparent_leader/ and move it out of loop.
We still have to check task_detached() in reparent_leader(), we can
find EXIT_DEAD leader.

Do you see any problems?

Oleg.

------------------------------------------------------------------------
[PATCH] do not place sub-threads on task_struct->children list

Currently we add sub-threads to ->real_parent->children list. This
buys nothing but slows down do_wait().

With this patch ->children contains only main threads (group leaders).
The only complication is that forget_original_parent() should iterate
over sub-threads by hand.

>From now do_wait_thread() can never see task_detached() && !EXIT_DEAD
tasks, we can remove this check (and we can unify do_wait_thread() and
ptrace_do_wait()).

This change can confuse the optimistic search inmm_update_next_owner(),
but this is fixable and minor.

Perhaps badness() and oom_kill_process() should be updated, but they
should be fixed in any case.

---

 kernel/fork.c |    2 +-
 kernel/exit.c |   41 ++++++++++++++++++++---------------------
 2 files changed, 21 insertions(+), 22 deletions(-)

--- WAIT/kernel/fork.c~3_CHILDREN_NO_THREADS	2009-06-30 00:57:16.000000000 +0200
+++ WAIT/kernel/fork.c	2009-06-30 00:57:50.000000000 +0200
@@ -1245,7 +1245,6 @@ static struct task_struct *copy_process(
 	}
 
 	if (likely(p->pid)) {
-		list_add_tail(&p->sibling, &p->real_parent->children);
 		tracehook_finish_clone(p, clone_flags, trace);
 
 		if (thread_group_leader(p)) {
@@ -1257,6 +1256,7 @@ static struct task_struct *copy_process(
 			p->signal->tty = tty_kref_get(current->signal->tty);
 			attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
 			attach_pid(p, PIDTYPE_SID, task_session(current));
+			list_add_tail(&p->sibling, &p->real_parent->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			__get_cpu_var(process_counts)++;
 		}
--- WAIT/kernel/exit.c~3_CHILDREN_NO_THREADS	2009-06-30 00:57:16.000000000 +0200
+++ WAIT/kernel/exit.c	2009-06-30 00:57:50.000000000 +0200
@@ -67,11 +67,11 @@ static void __unhash_process(struct task
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
 
+		list_del_init(&p->sibling);
 		list_del_rcu(&p->tasks);
 		__get_cpu_var(process_counts)--;
 	}
 	list_del_rcu(&p->thread_group);
-	list_del_init(&p->sibling);
 }
 
 /*
@@ -736,12 +736,9 @@ static struct task_struct *find_new_reap
 /*
 * Any that need to be release_task'd are put on the @dead list.
  */
-static void reparent_thread(struct task_struct *father, struct task_struct *p,
+static void reparent_leader(struct task_struct *father, struct task_struct *p,
 				struct list_head *dead)
 {
-	if (p->pdeath_signal)
-		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
-
 	list_move_tail(&p->sibling, &p->real_parent->children);
 
 	if (task_detached(p))
@@ -771,7 +768,7 @@ static void reparent_thread(struct task_
 
 static void forget_original_parent(struct task_struct *father)
 {
-	struct task_struct *p, *n, *reaper;
+	struct task_struct *g, *p, *n, *reaper;
 	LIST_HEAD(dead_children);
 
 	exit_ptrace(father);
@@ -779,13 +776,20 @@ static void forget_original_parent(struc
 	write_lock_irq(&tasklist_lock);
 	reaper = find_new_reaper(father);
 
-	list_for_each_entry_safe(p, n, &father->children, sibling) {
-		p->real_parent = reaper;
-		if (p->parent == father) {
-			BUG_ON(task_ptrace(p));
-			p->parent = p->real_parent;
-		}
-		reparent_thread(father, p, &dead_children);
+	list_for_each_entry_safe(g, n, &father->children, sibling) {
+		p = g;
+		do {
+			p->real_parent = reaper;
+			if (p->parent == father) {
+				BUG_ON(task_ptrace(p));
+				p->parent = p->real_parent;
+			}
+			if (p->pdeath_signal)
+				group_send_sig_info(p->pdeath_signal,
+						    SEND_SIG_NOINFO, p);
+		} while_each_thread(g, p);
+
+		reparent_leader(father, g, &dead_children);
 	}
 	write_unlock_irq(&tasklist_lock);
 
@@ -1533,14 +1537,9 @@ static int do_wait_thread(struct wait_op
 	struct task_struct *p;
 
 	list_for_each_entry(p, &tsk->children, sibling) {
-		/*
-		 * Do not consider detached threads.
-		 */
-		if (!task_detached(p)) {
-			int ret = wait_consider_task(wo, tsk, 0, p);
-			if (ret)
-				return ret;
-		}
+		int ret = wait_consider_task(wo, tsk, 0, p);
+		if (ret)
+			return ret;
 	}
 
 	return 0;