do_wait() optimization: do not place sub-threads on task_struct->children list

Thanks to Roland who pointed out de_thread() issues.

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, and de_thread() needs another list_replace() when it
changes ->group_leader.

Henceforth 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 in mm_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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ratan Nalumasu <rnalumasu@gmail.com>
Cc: Vitaly Mayatskikh <vmayatsk@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 18 insertions, 20 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 5962d7ccf243..546774a31a66 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -68,10 +68,10 @@ static void __unhash_process(struct task_struct *p)
                 detach_pid(p, PIDTYPE_SID);
 
                 list_del_rcu(&p->tasks);
+                list_del_init(&p->sibling);
                 __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_reaper(struct task_struct *father)
 /*
 * 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))
@@ -780,12 +777,18 @@ static void forget_original_parent(struct task_struct *father)
         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);
+                struct task_struct *t = p;
+                do {
+                        t->real_parent = reaper;
+                        if (t->parent == father) {
+                                BUG_ON(task_ptrace(t));
+                                t->parent = t->real_parent;
+                        }
+                        if (t->pdeath_signal)
+                                group_send_sig_info(t->pdeath_signal,
+                                                    SEND_SIG_NOINFO, t);
+                } while_each_thread(p, t);
+                reparent_leader(father, p, &dead_children);
         }
         write_unlock_irq(&tasklist_lock);
 
@@ -1551,14 +1554,9 @@ static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
         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, 0, p);
-                        if (ret)
-                                return ret;
-                }
+                int ret = wait_consider_task(wo, 0, p);
+                if (ret)
+                        return ret;
         }
 
         return 0;
diff --git a/kernel/fork.c b/kernel/fork.c
index 202a0ba63d3c..5b2959b3ffc2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1291,7 +1291,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         }
 
         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)) {
@@ -1303,6 +1302,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                         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)++;
                 }