Re: [RFC 09/16] kgr: mark task_safe in some kthreads

[Mike Galbraith]

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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