Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: dalias@libc.org, linux-sh@vger.kernel.org,
	catalin.marinas@arm.com, dave.hansen@linux.intel.com,
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Subject: Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware
Date: Tue, 24 Sep 2019 13:23:49 +0200
Message-ID: <20190924112349.GJ2332@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190924105622.GH23050@dhcp22.suse.cz>

On Tue, Sep 24, 2019 at 12:56:22PM +0200, Michal Hocko wrote:
> On Tue 24-09-19 11:17:14, Peter Zijlstra wrote:
> > On Tue, Sep 24, 2019 at 09:47:51AM +0200, Michal Hocko wrote:
> > > On Mon 23-09-19 22:34:10, Peter Zijlstra wrote:
> > > > On Mon, Sep 23, 2019 at 06:52:35PM +0200, Michal Hocko wrote:
> > > [...]
> > > > > I even the
> > > > > ACPI standard is considering this optional. Yunsheng Lin has referred to
> > > > > the specific part of the standard in one of the earlier discussions.
> > > > > Trying to guess the node affinity is worse than providing all CPUs IMHO.
> > > > 
> > > > I'm saying the ACPI standard is wrong.
> > > 
> > > Even if you were right on this the reality is that a HW is likely to
> > > follow that standard and we cannot rule out NUMA_NO_NODE being
> > > specified. As of now we would access beyond the defined array and that
> > > is clearly a bug.
> > 
> > Right, because the device node is wrong, so we fix _that_!
> > 
> > > Let's assume that this is really a bug for a moment. What are you going
> > > to do about that? BUG_ON? I do not really see any solution besides to either
> > > provide something sensible or BUG_ON. If you are worried about a
> > > conditional then this should be pretty easy to solve by starting the
> > > array at -1 index and associate it with the online cpu mask.
> > 
> > The same thing I proposed earlier; force the device node to 0 (or any
> > other convenient random valid value) and issue a FW_BUG message to the
> > console.
> 
> Why would you "fix" anything and how do you know that node 0 is the
> right choice? I have seen setups with node 0 without any memory and
> similar unexpected things.

We don't know 0 is right; but we know 'unkown' is wrong, so we FW_BUG
and pick _something_.

> To be honest I really fail to see why to object to a simple semantic
> that NUMA_NO_NODE imply all usable cpus. Could you explain that please?

Because it feels wrong. The device needs to be _somewhere_. It simply
cannot be node-less.