Date:	Tue, 26 Oct 2010 08:47:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

On Tue, Oct 26, 2010 at 12:29 AM, Mike Galbraith <mgalbraith@...e.de> wrote:
> On Tue, 2010-10-26 at 09:07 +0200, Mike Galbraith wrote:
>> On Thu, 2010-10-21 at 18:29 +0200, Oleg Nesterov wrote:
>>
>> > It is not clear to me why do we need rcu_read_lock() and how it can help.
>> > The tty can go away right after dereferencing signal->tty.
>>
>> Which was Marcus' crash.  Didn't happen here only because I didn't have
>> CONFIG_PREEMPT set.
>>
>> Changes since v2:
>>   - drop
>
> Bumped mouse, message escaped.
>
> Doesn't matter though, damn thing just blew up during enable/disable
> plus hackbench stress test, despite holding a reference to the tty at
> every place tty changes (under sighand lock), and moving the task with
> that reference held.

So I have a suggestion that may not be popular with you, because it
does end up changing the approach of your patch a lot.

And I have to say, I like how your last patch looked. It was
surprisingly small, simple, and clean. So I hate saying "I think it
should perhaps do things a bit differently". That said, I would
suggest:

 - don't depend on "tsk->signal->tty" at all.

 - INSTEAD, introduce a "tsk->signal->sched_group" pointer that points
to whatever the current auto-task_group is. Remember, long-term, we'd
want to maybe have other heuristics than just the tty groups, so we'd
want this separate from the tty logic _anyway_

 - at fork time, just copy the task_group pointer in copy_signal() if
it is non-NULL, and increment the refcount (I don't think struct
task_group is refcounted now, but this would require it).

 - at free_signal_struct(), just do a
"put_task_group(sig->task_group);" before freeing it.

 - make the scheduler use the "tsk->signal->sched_group" as the
default group if nothing else exists.

Now, all the basic logic is _entirely_ unaware of any tty logic, and
it's generic. And none of it has any races with some odd tty release
logic or anything like that.

Now, after this, the only thing you'd need to do is hook into
__proc_set_tty(), which already holds the sighand lock, and _there_
you would attach the task_group to the process. Notice how it would
never be attached to a tty at all, so tty_release etc would never be
involved in any taskgroup thing - it's not really the tty that owns
the taskgroup, it's simply the act of becoming a tty task group leader
that attaches the task to a new scheduling group.

It also means, for example, that if a process loses its tty (and
doesn't get a new one - think hangup), it still remains in whatever
scheduling group it started out with. The tty really is immaterial.

And the nice thing about this is that it should be trivial to make
other things than tty's trigger this same thing, if we find a pattern
(or create some new interface to let people ask for it) for something
that should create a new group (like perhaps spawning a graphical
application from the window manager rather than from a tty).

Comments?

                                           Linus
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