Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
From: Mike Galbraith <efault@gmx.de>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <20101116130413.GA29368@redhat.com>
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Date: Tue, 16 Nov 2010 07:18:29 -0700
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On Tue, 2010-11-16 at 14:04 +0100, Oleg Nesterov wrote:
> 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.

Is _not_ needed I presume.

I'll remove it, I'm not overly attached (a t t a..;) to it. 

> 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().

(yeah)

> > +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).

My mouse copied it.

> 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.

I'll do it that way.  I was pondering adding the option to move one or
all as cgroups does, but don't think that will ever be needed.

	Thanks,

	-Mike

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