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From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH] sched: Relax a restriction in sched_rt_can_attach()
Date: Tue, 5 May 2015 15:06:03 -0400
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To: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Cc: Zefan Li <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>, Mike Galbraith <umgwanakikbuti-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>

Hello, Peter.

On Tue, May 05, 2015 at 09:00:57PM +0200, Peter Zijlstra wrote:
> On Tue, May 05, 2015 at 12:31:12PM -0400, Tejun Heo wrote:
> > What I don't want to happen is controllers failing migrations
> > willy-nilly for random reasons leaving users baffled, which we've
> > actually been doing unfortunately.  Maybe we need to deal with this
> > fixed resource arbitration as a separate class and allow them to fail
> > migration w/ -EBUSY.
> 
> Ah, _that_ was the problem.
> 
> Which is something created by this co-mounting of controllers.

Yeah, partly, but also that it's an extra failure mode which isn't
necessary for most controllers.

> You could of course store the ss-id of the failing operation in
> task_struct and have a file reporting the name of the ss-id.
> 
> That way, there is a simple way to find out which controller failed the
> migrate.

Given that the resources which can fail are very limited, I don't
think we need that right now as long as we limit and document the
possible failure cases clearly.  Hopefully, this won't devolve into
collection of arbitrary failures.

Thanks.

-- 
tejun