Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

[Oleg Nesterov <oleg@redhat.com>]

On 08/15, Peter Zijlstra wrote:
>
> Also; why do we care about PROCESS_CPUTIME? People should really not use
> it. What are the 'valid' usecases you guys care about?

I do not really know. IIUC, the problematic usecase is sys_times().

I agree with Mike, "don't do this if you have a lot of threads". But
perhaps the kernel can help to applications which already abuse times().

However, if we only want to make sys_times() more scalable(), then
perhaps the "lockless" version of thread_group_cputime() makes more
sense. And given that do_sys_times() uses current we can simplify it;
is_dead is not possible and we do not need to take ->siglock twice:

	void current_group_cputime(struct task_cputime *times)
	{
		struct task_struct *tsk = current, *t;
		struct spinlock_t *siglock = &tsk->sighand->siglock;
		struct signal_struct *sig = tsk->signal;
		bool lockless = true;
		u64 exec;

	 retry:
		spin_lock_irq(siglock);
		times->utime = sig->utime;
		times->stime = sig->stime;
		times->sum_exec_runtime = exec = sig->sum_sched_runtime;

		if (lockless)
			spin_unlock_irq(siglock);

		rcu_read_lock();
		for_each_thread(tsk, t) {
			cputime_t utime, stime;
			task_cputime(t, &utime, &stime);
			times->utime += utime;
			times->stime += stime;
			times->sum_exec_runtime += task_sched_runtime(t);
		}
		rcu_read_unlock();

		if (lockless) {
			lockless = false;
			spin_unlock_wait(siglock);
			smp_rmb();
			if (exec != sig->sum_sched_runtime)
				goto retry;
		} else {
			spin_unlock_irq(siglock);
		}
	}

Oleg.