linux-kernel - Re: [RFC 09/16] kgr: mark task

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Message-ID: <1400130262.5175.93.camel@marge.simpson.net>
Date:	Thu, 15 May 2014 07:04:22 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vojtech Pavlik <vojtech@...e.cz>, Jiri Slaby <jslaby@...e.cz>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	jirislaby@...il.com, Michael Matz <matz@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC 09/16] kgr: mark task_safe in some kthreads

On Thu, 2014-05-15 at 00:50 -0400, Tejun Heo wrote: 
> Hello, Mike.
> 
> On Thu, May 15, 2014 at 06:46:18AM +0200, Mike Galbraith wrote:
> > > I think it'd be healthier to identify the use cases and then provide
> > > proper interface for it.  Note that workqueue can now expose interface
> > > to modify concurrency, priority and cpumask to userland which
> > > writeback workers are already using.
> > 
> > You can't identify a specific thing, any/all of it can land on the
> > user's diner plate, so he should be able to make the decisions.  Power
> > to the user and all that, if he does something stupid, tuff titty.  User
> > getting to call the shots, and getting to keep the pieces when he fscks
> > it all up is wonderful stuff, lets kernel people off the hook :)
> 
> Do we know specific kthreads which need to be exposed with this way?

Soft/hard irq threads and anything having to do with IO mostly, which
including workqueues.  I had to give the user a rather fugly global
prioritization option to let users more or less safely do the evil deeds
they want to and WILL do whether I agree with their motivation to do so
or not.  I tell all users that realtime is real dangerous, but if they
want to do that, it's their box, so by definition perfectly fine.

> If there are good enough reasons for specific ones, sure, but I don't
> think "we can't change any of the kthreads because someone might be
> diddling with it" is something we can sustain in the long term.

I think the opposite.  Taking any control the user has is pure evil.

-Mike

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