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From: Michal Hocko <mhocko@kernel.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, 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
Date: Wed, 29 Apr 2020 14:22:11 +0200
Message-ID: <20200429122211.GD28637@dhcp22.suse.cz> (raw)
In-Reply-To: <20200429014145.GD19958@linux.vnet.ibm.com>

On Wed 29-04-20 07:11:45, Srikar Dronamraju wrote:
> > > 
> > > 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.

Have you tested on something else than ppc? Each arch does the NUMA
setup separately and this is a big mess. E.g. x86 marks even memory less
nodes (see init_memory_less_node) as online.

Honestly I have hard time to evaluate the effect of this patch. It makes
some sense to assume all nodes offline before they get online but this
is a land mine territory.

I am also not sure what kind of problem this is going to address. You
have mentioned numa balancing without many details.
-- 
Michal Hocko
SUSE Labs

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-28  9:38 [PATCH v2 0/3] Offline memoryless cpuless node 0 Srikar Dronamraju
2020-04-28  9:38 ` [PATCH v2 1/3] powerpc/numa: Set numa_node for all possible cpus Srikar Dronamraju
2020-04-28  9:38 ` [PATCH v2 2/3] powerpc/numa: Prefer node id queried from vphn Srikar Dronamraju
2020-04-29  6:52   ` Gautham R Shenoy
2020-04-30  4:34     ` Srikar Dronamraju
2020-04-28  9:38 ` [PATCH v2 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline Srikar Dronamraju
2020-04-28 23:59   ` Andrew Morton
2020-04-29  1:41     ` Srikar Dronamraju
2020-04-29 12:22       ` Michal Hocko [this message]
2020-04-30  7:18         ` Srikar Dronamraju
2020-05-04  9:37           ` Michal Hocko
2020-05-08 13:03             ` Srikar Dronamraju
2020-05-08 13:39               ` David Hildenbrand
2020-05-08 13:42                 ` David Hildenbrand
2020-05-11 17:47                   ` Srikar Dronamraju
2020-05-12  7:49                     ` David Hildenbrand
2020-05-12 10:42                       ` Srikar Dronamraju

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