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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 15 Nov 2010 18:18:20 -0800
Message-ID: <AANLkTik4J7u1zC+D7bkB5PyVHRS_pLOU=c5HFUMxGqwM@mail.gmail.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>, Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
        Balbir Singh <balbir@linux.vnet.ibm.com>
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On Mon, Nov 15, 2010 at 5:56 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
>
> Should this kind of thing be done in user space?

Almost certainly not.

First off, user-space is a fragmented mess. Just from a "let's get it
done" angle, it just doesn't work. There are lots of different thing
that create new tty's, and you can't have them all fixed. Plus it
would be _way_ more code in user space than it is in kernel space.

Secondly, user-space daemons are a total mess. We've tried it many
many times, and every time the _intention_ is to make things simpler
to debug and deploy. And it almost never is. The interfaces end up
being too painful, and the "part of the code is in kernel space, part
of it is in user space" means that things just break all the time.

Finally, the whole "user space is more flexible" is just a lie. It
simply doesn't end up being true. It will be _harder_ to configure
some user-space daemon than it is to just set a flag in /sys or
whatever. The "flexibility" tends to be more a flexibility to get
things wrong than any actual advantage.

Just look at the patch in question. It's simple, it's clean, and it
"just works". Doing the same thing in user space? It would be a total
nightmare, and exactly _because_ it would be a total nightmare, the
code would never be that simple or clean.

                       Linus
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