Re: [PATCH RFC] time: drop do_sys_times spinlock

[Oleg Nesterov <oleg@redhat.com>]

On 08/13, Rik van Riel wrote:
>
> On Wed, 13 Aug 2014 19:22:30 +0200
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > On 08/12, Rik van Riel wrote:
> > >
> > > Any other ideas?
> >
> > To simplify, lets suppose that we only need sum_exec_runtime.
> >
> > Perhaps we can do something like this
>
> That would probably work, indeed.

OK, perhaps I'll try to make a patch tomorrow for review.

> However, it turns out that a seqcount doesn't look too badly either.

Well, I disagree. This is more complex, and this adds yet another lock
which only protects the stats...

> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -461,6 +461,7 @@ struct sighand_struct {
>  	atomic_t		count;
>  	struct k_sigaction	action[_NSIG];
>  	spinlock_t		siglock;
> +	seqcount_t		stats_seq; /* write nests inside spinlock */

No, no, at least it should go to signal_struct. Unlike ->sighand, ->signal
is stable as long as task_struct can't go away.

>  void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>  {
>  	struct signal_struct *sig = tsk->signal;
> +	struct sighand_struct *sighand;
>  	cputime_t utime, stime;
>  	struct task_struct *t;
> -
> -	times->utime = sig->utime;
> -	times->stime = sig->stime;
> -	times->sum_exec_runtime = sig->sum_sched_runtime;
> +	int seq;
>  
>  	rcu_read_lock();
> -	/* make sure we can trust tsk->thread_group list */
> -	if (!likely(pid_alive(tsk)))
> +	sighand = rcu_dereference(tsk->sighand);
> +	if (unlikely(!sighand))
>  		goto out;
>  
> -	t = tsk;
>  	do {
> -		task_cputime(t, &utime, &stime);
> -		times->utime += utime;
> -		times->stime += stime;
> -		times->sum_exec_runtime += task_sched_runtime(t);
> -	} while_each_thread(tsk, t);
> +		seq = read_seqcount_begin(&sighand->stats_seq);
> +		times->utime = sig->utime;
> +		times->stime = sig->stime;
> +		times->sum_exec_runtime = sig->sum_sched_runtime;
> +
> +		/* make sure we can trust tsk->thread_group list */
> +		if (!likely(pid_alive(tsk)))
> +			goto out;

Whatever we do, we should convert thread_group_cputime() to use
for_each_thread() first().

> @@ -781,14 +781,14 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
>  		cpu_clock_sample(timer->it_clock, p, &now);
>  	} else {
>  		struct sighand_struct *sighand;
> -		unsigned long flags;
>
>  		/*
>  		 * Protect against sighand release/switch in exit/exec and
>  		 * also make timer sampling safe if it ends up calling
>  		 * thread_group_cputime().
>  		 */
> -		sighand = lock_task_sighand(p, &flags);
> +		rcu_read_lock();
> +		sighand = rcu_dereference(p->sighand);

This looks unneeded at first glance.

Oleg.