linux-kernel - Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

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Message-ID: <20101116130413.GA29368@redhat.com>
Date:	Tue, 16 Nov 2010 14:04:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

On 11/15, Mike Galbraith wrote:
>
> On Mon, 2010-11-15 at 13:57 +0100, Oleg Nesterov wrote:
>
> > And the exiting task can do a lot before it disappears, probably
> > we shouldn't ignore ->autogroup.

I don't really understand what makes the exiting task different,
but OK.

However, I must admit I dislike this check. Because, looking at this
code, it is not clear why do we check PF_EXITING. It looks as if it
is needed for correctness.

OK, this is minor. I think the patch is correct, just one nit below.

> > It can't protect the change of signal->autogroup, multiple callers
> > can use different rq's.
>
> Guaranteed live ->autogroup should be good enough for heuristic use, and
> had better be so.  Having to take ->siglock in the fast path would kill
> using ->signal.

Yes, sure, rq->lock should ensure signal->autogroup can't go away.
(even if it can be changed under us). And it does, we are moving all
threads before kref_put().

> +static void
> +autogroup_move_group(struct task_struct *p, struct autogroup *ag)
> +{
> +	struct autogroup *prev;
> +	struct task_struct *t;
> +
> +	prev = p->signal->autogroup;
> +	if (prev == ag)
> +		return;
> +
> +	p->signal->autogroup = autogroup_kref_get(ag);
> +	sched_move_task(p);
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(t, &p->thread_group, thread_group) {
> +		sched_move_task(t);
> +	}
> +	rcu_read_unlock();
> +
> +	autogroup_kref_put(prev);
> +}

Well, this looks a bit strange (but correct).

We are changing ->autogroup assuming the caller holds ->siglock.
But if we hold ->siglock we do not need rcu_read_lock() to iterate
over the thread_group, we can just do

	p->signal->autogroup = autogroup_kref_get(ag);

	t = p;
	do {
		sched_move_task(t);
	} while_each_thread(p, t);

Again, this is minor, I won't insist.

Oleg.

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