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>
diff --git a/fs/exec.c b/fs/exec.c
index 623a5cc..77db9a9 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -826,7 +826,9 @@
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;
