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


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