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From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        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>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline
Date:   Wed, 11 Mar 2020 16:32:37 +0530
In-Reply-To: <20200311110237.5731-1-srikar@linux.vnet.ibm.com>
References: <20200311110237.5731-1-srikar@linux.vnet.ibm.com>
Message-Id: <20200311110237.5731-4-srikar@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Currently Linux kernel with CONFIG_NUMA on a system with multiple
possible nodes, marks node 0 as online at boot.  However in practice,
there are systems which have node 0 as memoryless and cpuless.

This can cause numa_balancing to be enabled on systems with only one node
with memory and CPUs. The existence of this dummy node which is cpuless and
memoryless node can confuse users/scripts looking at output of lscpu /
numactl.

Lets stop assuming that Node 0 is always online.

v5.6-rc4
 available: 2 nodes (0,2)
 node 0 cpus:
 node 0 size: 0 MB
 node 0 free: 0 MB
 node 2 cpus: 0 1 2 3 4 5 6 7
 node 2 size: 32625 MB
 node 2 free: 31490 MB
 node distances:
 node   0   2
   0:  10  20
   2:  20  10

proc and sys files
------------------
 /sys/devices/system/node/online:            0,2
 /proc/sys/kernel/numa_balancing:            1
 /sys/devices/system/node/has_cpu:           2
 /sys/devices/system/node/has_memory:        2
 /sys/devices/system/node/has_normal_memory: 2
 /sys/devices/system/node/possible:          0-31

v5.6-rc4 + patch
------------------
 available: 1 nodes (2)
 node 2 cpus: 0 1 2 3 4 5 6 7
 node 2 size: 32625 MB
 node 2 free: 31487 MB
 node distances:
 node   2
   2:  10

proc and sys files
------------------
/sys/devices/system/node/online:            2
/proc/sys/kernel/numa_balancing:            0
/sys/devices/system/node/has_cpu:           2
/sys/devices/system/node/has_memory:        2
/sys/devices/system/node/has_normal_memory: 2
/sys/devices/system/node/possible:          0-31

Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Christopher Lameter <cl@linux.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb75..68e635f4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -116,8 +116,10 @@ struct pcpu_drain {
  */
 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 } },
-- 
1.8.3.1