linux-kernel - Re: [PATCH 2/2] s390/topology: add drawer scheduling domain level

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Date:	Mon, 13 Jun 2016 15:53:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] s390/topology: add drawer scheduling domain level

On Mon, Jun 13, 2016 at 03:19:42PM +0200, Martin Schwidefsky wrote:
> On Mon, 13 Jun 2016 15:06:47 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, Jun 13, 2016 at 01:22:30PM +0200, Heiko Carstens wrote:
> > > Yes, and actually we are all virt/LPAR always, so this is unfortunately not
> > > very easy to do. And yes, I do agree that for the 1:1 case it most likely
> > > would make sense, however we don't have any run-time guarantee to stay 1:1.
> > 
> > One option would be to make it a boot option; such that the
> > administrator has to set it. At that point, if the admin creates
> > multiple LPARs its on him.
> 
> Unfortunately not good enough. The LPAR code tries to optimize the layout
> at the time a partition is activated. The landscape of already running
> partitions can change at this point.

Would not the admin _know_ this? It would be him activating partitions
after all, no?

> To get around this you would have to activate *all* partitions first and
> then start the operating systems in a second step.

Arguably, you only care about the single partition covering the entire
machine case, so I don't see that being a problem.

Again, admin _knows_ this.

> And then there is concurrent repair which will move things around if a
> piece of memory goes bad. This happens rarely though.

That would be magic disturbance indeed, nothing much to do about that.