Re: [PATCH v2 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline

Andrew Morton
2020-04-28T23:59:15Z

On Tue, 28 Apr 2020 15:08:36 +0530 Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> 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.
>
> 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?