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Commit 9cd80bbb authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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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: default avatarOleg 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0f67b0b0

fs/exec.c

+2 −0
Original line number Diff line number Diff line
@@ -826,7 +826,9 @@ static int de_thread(struct task_struct *tsk) 
		attach_pid(tsk, PIDTYPE_PID,  task_pid(leader));

		transfer_pid(leader, tsk, PIDTYPE_PGID);

		transfer_pid(leader, tsk, PIDTYPE_SID);



		list_replace_rcu(&leader->tasks, &tsk->tasks);

		list_replace_init(&leader->sibling, &tsk->sibling);



		tsk->group_leader = tsk;

		leader->group_leader = tsk;

kernel/exit.c

+17 −19
Original line number Diff line number Diff line
@@ -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;

		struct task_struct *t = p;

		do {

			t->real_parent = reaper;

			if (t->parent == father) {

				BUG_ON(task_ptrace(t));

				t->parent = t->real_parent;

			}

		reparent_thread(father, p, &dead_children);

			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);
@@ -1551,15 +1554,10 @@ 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;

	}

	}



	return 0;

}

kernel/fork.c

+1 −1
Original line number Diff line number Diff line
@@ -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)++;

		}