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Date:   Wed, 29 Apr 2020 07:11:45 +0530
From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Christopher Lameter <cl@linux.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline
Message-ID: <20200429014145.GD19958@linux.vnet.ibm.com>
Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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> > 
> > By marking, N_ONLINE as NODE_MASK_NONE, lets stop assuming that Node 0 is
> > always online.
> > 
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -116,8 +116,10 @@ EXPORT_SYMBOL(latent_entropy);
> >   */
> >  nodemask_t node_states[NR_NODE_STATES] __read_mostly = {
> >  	[N_POSSIBLE] = NODE_MASK_ALL,
> > +#ifdef CONFIG_NUMA
> > +	[N_ONLINE] = NODE_MASK_NONE,
> > +#else
> >  	[N_ONLINE] = { { [0] = 1UL } },
> > -#ifndef CONFIG_NUMA
> >  	[N_NORMAL_MEMORY] = { { [0] = 1UL } },
> >  #ifdef CONFIG_HIGHMEM
> >  	[N_HIGH_MEMORY] = { { [0] = 1UL } },
> 
> So on all other NUMA machines, when does node 0 get marked online?
> 
> This change means that for some time during boot, such machines will
> now be running with node 0 marked as offline.  What are the
> implications of this?  Will something break?

Till the nodes are detected, marking Node 0 as online tends to be redundant.
Because the system doesn't know if its a NUMA or a non-NUMA system.
Once we detect the nodes, we online them immediately. Hence I don't see any
side-effects or negative implications of this change.

However if I am missing anything, please do let me know.

From my part, I have tested this on
1. Non-NUMA Single node but CPUs and memory coming from zero node.
2. Non-NUMA Single node but CPUs and memory coming from non-zero node.
3. NUMA Multi node but with CPUs and memory from node 0.
4. NUMA Multi node but with no CPUs and memory from node 0.

-- 
Thanks and Regards
Srikar Dronamraju