[PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

We run some sub-cases (fork, exec, pipe, tcp, udp) of aim7 on 8-socket machine.
Perf shows write_lock_irq(&tasklist_lock) consumes more than 50% cpu time.

One hot caller is exit_ptrace. If the exiting process doesn't ptrace other
processes, kernel needn't apply for the write lock on tasklist_lock.

With below patch against kernel 2.6.35-rc5, we get more than 10% result improvement.

Signed-off-by: Zhang Yanmin <yanmin_zhang@linux.intel.com>

---

diff -Nraup linux-2.6.35-rc5/kernel/ptrace.c linux-2.6.35-rc5_ptrace/kernel/ptrace.c
--- linux-2.6.35-rc5/kernel/ptrace.c	2010-07-16 14:01:15.000000000 +0800
+++ linux-2.6.35-rc5_ptrace/kernel/ptrace.c	2010-07-16 14:03:20.000000000 +0800
@@ -331,6 +331,9 @@ void exit_ptrace(struct task_struct *tra
 	struct task_struct *p, *n;
 	LIST_HEAD(ptrace_dead);
 
+	if (list_empty(&tracer->ptraced))
+		return;
+
 	write_lock_irq(&tasklist_lock);
 	list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
 		if (__ptrace_detach(tracer, p))