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Subject[PATCH -mm 1/3] exit: reparent: avoid find_new_reaper() if no children
Now that pid_ns logic was isolated we can change forget_original_parent()
to return right after find_child_reaper() when father->children is empty,
there is nothing to reparent in this case.

In particular this avoids find_alive_thread() and this can help if the
whole process exits and it has a lot of PF_EXITING threads at the start
of the thread list, this can easily lead to O(nr_threads ** 2) iterations.

Trivial test case (tested under KVM, 2 CPUs):

static void *tfunc(void *arg)
{
pause();
return NULL;
}

static int child(unsigned int nt)
{
pthread_t pt;

while (nt--)
assert(pthread_create(&pt, NULL, tfunc, NULL) == 0);

pthread_kill(pt, SIGTRAP);
pause();
return 0;
}

int main(int argc, const char *argv[])
{
int stat;
unsigned int nf = atoi(argv[1]);
unsigned int nt = atoi(argv[2]);

while (nf--) {
if (!fork())
return child(nt);

wait(&stat);
assert(stat == SIGTRAP);
}

return 0;
}

$ time ./test 16 16536 shows:

real user sys
- 5m37.628s 0m4.437s 8m5.560s
+ 0m50.032s 0m7.130s 1m4.927s

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/exit.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 4e3475d..81b62f8 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -571,6 +571,8 @@ static void forget_original_parent(struct task_struct *father)

/* Can drop and reacquire tasklist_lock */
reaper = find_child_reaper(father);
+ if (list_empty(&father->children))
+ goto unlock;

reaper = find_new_reaper(father, reaper);
list_for_each_entry(p, &father->children, sibling) {
@@ -591,6 +593,7 @@ static void forget_original_parent(struct task_struct *father)
reparent_leader(father, p, &dead_children);
}
list_splice_tail_init(&father->children, &reaper->children);
+ unlock:
write_unlock_irq(&tasklist_lock);

list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {
--
1.5.5.1


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