Re: [PATCH] ptrace: cleanup ptrace_init_task()->ptrace_link() path

[Oleg Nesterov <oleg@redhat.com>]

On 10/30, Andrew Morton wrote:
>
> On Fri, 30 Oct 2009 00:56:56 +0100
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> >  ptrace
>
> Speaking of which, I'm still sitting on
> do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list.patch.

(this patch has nothing to do with ptrace)

> Should I drop it?

Why? I think this is good optimization and imho cleanup.

There is no point to have sub-thread in ->children list and this
slows down do_wait() if a child has a lot of threads, it has to
iterate over all sub-threads just to filter them out.

Oleg.

> From: Oleg Nesterov <oleg@redhat.com>
>
> 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>
> ---
>
>  fs/exec.c     |    2 ++
>  kernel/exit.c |   36 +++++++++++++++++-------------------
>  kernel/fork.c |    2 +-
>  3 files changed, 20 insertions(+), 20 deletions(-)
>
> diff -puN fs/exec.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list fs/exec.c
> --- a/fs/exec.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list
> +++ a/fs/exec.c
> @@ -829,7 +829,9 @@ static int de_thread(struct task_struct
>  		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;
> diff -puN kernel/exit.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list kernel/exit.c
> --- a/kernel/exit.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list
> +++ a/kernel/exit.c
> @@ -67,10 +67,10 @@ static void __unhash_process(struct task
>  		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);
>  }
>
>  /*
> @@ -737,12 +737,9 @@ 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))
> @@ -781,12 +778,18 @@ static void forget_original_parent(struc
>  	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);
>
> @@ -1546,14 +1549,9 @@ 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, 0, p);
> -			if (ret)
> -				return ret;
> -		}
> +		int ret = wait_consider_task(wo, 0, p);
> +		if (ret)
> +			return ret;
>  	}
>
>  	return 0;
> diff -puN kernel/fork.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list kernel/fork.c
> --- a/kernel/fork.c~do_wait-optimization-do-not-place-sub-threads-on-task_struct-children-list
> +++ a/kernel/fork.c
> @@ -1273,7 +1273,6 @@ 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)) {
> @@ -1285,6 +1284,7 @@ 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)++;
>  		}
> _
>