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Date: Sat, 31 Oct 2009 01:55:07 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Roland McGrath <roland@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ptrace: cleanup ptrace_init_task()->ptrace_link() path
On 10/30, Andrew Morton wrote:
>
> On Fri, 30 Oct 2009 00:56:56 +0100
> Oleg Nesterov <oleg@...hat.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@...hat.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@...hat.com>
> Cc: Roland McGrath <roland@...hat.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Ratan Nalumasu <rnalumasu@...il.com>
> Cc: Vitaly Mayatskikh <vmayatsk@...hat.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Signed-off-by: Andrew Morton <akpm@...ux-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)++;
> }
> _
>
--
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