linux-kernel - Re: [PATCH v4 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline

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Message-ID: <20200513074612.GA22908@linux.vnet.ibm.com>
Date:   Wed, 13 May 2020 13:16:12 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline

* Christopher Lameter <cl@...ux.com> [2020-05-12 16:31:26]:

> On Tue, 12 May 2020, Srikar Dronamraju wrote:
> 
> > +#ifdef CONFIG_NUMA
> > +	[N_ONLINE] = NODE_MASK_NONE,
> 
> Again. Same issue as before. If you do this then you do a global change
> for all architectures. You need to put something in the early boot
> sequence (in a non architecture specific way) that sets the first node
> online by default.
> 

I did respond to that earlier.

> You have fixed the issue in your earlier patches for the powerpc
> archicture. What about the other architectures?
> 
> Or did I miss something?
> 

Here are my assumptions, please do correct me if any of them are wrong.
1. My other patches for Powerpc, don't change when the nodes are being
onlined. They only change how the cpu_to_node numbering of the offline cpus.
In this respect Powerpc due to its PAPR compliance may be slightly unique
from other archs where the cpu binding of the node is not known till CPUs
are onlined.

2. Currently the nodes are onlined (in all arch specific code) as soon as
they are detected. This is unconditional onlining as in there are no checks
to see the node number is 0. i.e I don't see any special checks that
restrict or allow node 0 from being onlined / offlined. Its considered no
special than any other online node.

3. If we were to expect node 0 to be always online, then why do we have
first_online_node. We could always hard code it to 0.

4. I tried enabling CONFIG_MEMORYLESS_NODE on x86, but that's seems to be
not possible. And it looks to me that something like that is only possible
on powerpc and IA64.

5. Without my patch on a regular numa system, node 0 would be onlined by
default during structure initialization. When the nodes get detected, node 0
and other nodes would again be onlined. The only drawback being if node 0
wasn't suppose to be online, it will still end up being marked online.
With the proposed patch, when the nodes get detected, any nodes detected
would be onlined.

I think the node onlining is already pretty early in boot. I don't know of
any other mechanism to move the onlining further up and in a non
architecture specific way. However if you have ideas, please do let me know.

-- 
Thanks and Regards
Srikar Dronamraju