alinux: kernel: Reduce tasklist_lock contention at fork and exit

to #16966300
to #16966377

We observed heavy tasklist_lock contention when offline tasks
start and end, which caused long scheduling latency and long
period with interrupt off since write_lock_irq() is called.

In extreme cases with tons of concurrent fork and exit events,
it can cause the system hung.

This patch changed them to use trylock, then operations within
lock consume little time, so it naturally addresses the issue.

After this patch, when I launched and killed thousands of tasks,
the latency can reduce from tens of milliscends to around 2ms on
my box.

The patch can pass Unixbench tests, no regression introduced.
Reviewed-by: NShanpei Chen <shanpeic@linux.alibaba.com>
Signed-off-by: NXunlei Pang <xlpang@linux.alibaba.com>
Acked-by: NShile Zhang <shile.zhang@linux.alibaba.com>

kernel/exit.c

@@ -710,10 +710,17 @@ static void forget_original_parent(struct task_struct *father,
 static void exit_notify(struct task_struct *tsk, int group_dead)
 {
 	bool autoreap;
+	unsigned long flags;
 	struct task_struct *p, *n;
 	LIST_HEAD(dead);
 
-	write_lock_irq(&tasklist_lock);
+retry:
+	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
+		if (!in_atomic() && !irqs_disabled())
+			cond_resched();
+		goto retry;
+	}
+
 	forget_original_parent(tsk, &dead);
 
 	if (group_dead)

kernel/fork.c

@@ -1658,6 +1658,7 @@ static __latent_entropy struct task_struct *copy_process(
 	int retval;
 	struct task_struct *p;
 	struct multiprocess_signals delayed;
+	unsigned long flags;
 
 	/*
 	 * Don't allow sharing the root directory with processes in a different
@@ -1986,7 +1987,11 @@ static __latent_entropy struct task_struct *copy_process(
 	 * Make it visible to the rest of the system, but dont wake it up yet.
 	 * Need tasklist lock for parent etc handling!
 	 */
-	write_lock_irq(&tasklist_lock);
+retry:
+	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
+		cond_resched();
+		goto retry;
+	}
 
 	/* CLONE_PARENT re-uses the old parent */
 	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {