From ad9e206aefa56788b676ebcd6329e828f40d2238 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 10 Dec 2014 15:55:17 -0800 Subject: [PATCH] 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 Cc: Aaron Tomlin Cc: "Eric W. Biederman" Cc: Sterling Alexander Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/exit.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/kernel/exit.c b/kernel/exit.c index b0f482f..0637456 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) { -- 2.7.4