Re: [PATCH] powerpc/numa: fix hot-added CPU on memory-less node

[Satheesh Rajendran]

On Wed, Nov 14, 2018 at 06:03:19PM +0100, Laurent Vivier wrote:
> Trying to hotplug a CPU on an empty NUMA node (without
> memory or CPU) crashes the kernel when the CPU is onlined.
> 
> During the onlining process, the kernel calls start_secondary()
> that ends by calling
> set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]))
> that relies on NODE_DATA(nid)->node_zonelists and in our case
> NODE_DATA(nid) is NULL.
> 
> To fix that, add the same checking as we already have in
> find_and_online_cpu_nid(): if NODE_DATA() is NULL, use
> the first online node.
> 
> Bug: https://github.com/linuxppc/linux/issues/184
> Fixes: ea05ba7c559c8e5a5946c3a94a2a266e9a6680a6
>        (powerpc/numa: Ensure nodes initialized for hotplug)
> Signed-off-by: Laurent Vivier <lvivier@xxxxxxxxxx>
> ---
>  arch/powerpc/mm/numa.c | 9 +++++++++
>  1 file changed, 9 insertions(+)

This patch causes regression for cold plug numa case(Case 1) and 
hotplug case + reboot(Case 2) with adding all vcpus into node 0.


Env: HW: Power8 Host.
Kernel: 4.20-rc2 + this patch

Case 1:
1. boot a guest with 8 vcpus(all available), spreadout in 4 numa nodes.
<vcpu placement='static'>8</vcpu>
...
   <numa>
      <cell id='0' cpus='0-1' memory='4194304' unit='KiB'/>
      <cell id='1' cpus='2-3' memory='4194304' unit='KiB'/>
      <cell id='2' cpus='4-5' memory='0' unit='KiB'/>
      <cell id='3' cpus='6-7' memory='0' unit='KiB'/>
    </numa>

2. Check lscpu --- all vcpus are added to node0 --> NOK

# lscpu
Architecture:        ppc64le
Byte Order:          Little Endian
CPU(s):              8
On-line CPU(s) list: 0-7
Thread(s) per core:  1
Core(s) per socket:  8
Socket(s):           1
NUMA node(s):        4
Model:               2.1 (pvr 004b 0201)
Model name:          POWER8 (architected), altivec supported
Hypervisor vendor:   KVM
Virtualization type: para
L1d cache:           64K
L1i cache:           32K
NUMA node0 CPU(s):   0-7
NUMA node1 CPU(s):   
NUMA node2 CPU(s):   
NUMA node3 CPU(s): 

without this patch it was working fine.
# lscpu
Architecture:        ppc64le
Byte Order:          Little Endian
CPU(s):              8
On-line CPU(s) list: 0-7
Thread(s) per core:  1
Core(s) per socket:  8
Socket(s):           1
NUMA node(s):        4
Model:               2.1 (pvr 004b 0201)
Model name:          POWER8 (architected), altivec supported
Hypervisor vendor:   KVM
Virtualization type: para
L1d cache:           64K
L1i cache:           32K
NUMA node0 CPU(s):   0,1
NUMA node1 CPU(s):   2,3
NUMA node2 CPU(s):   4,5
NUMA node3 CPU(s):   6,7


Case 2:
1. boot a guest with 8 vcpus(2 available, 6 possible), spreadout in 4 numa nodes.
<vcpu placement='static' current='2'>8</vcpu>
...
   <numa>
      <cell id='0' cpus='0-1' memory='0' unit='KiB'/>
      <cell id='1' cpus='2-3' memory='4194304' unit='KiB'/>
      <cell id='2' cpus='4-5' memory='0' unit='KiB'/>
      <cell id='3' cpus='6-7' memory='0' unit='KiB'/>
    </numa>

2. Hotplug all vcpus
# lscpu
Architecture:        ppc64le
Byte Order:          Little Endian
CPU(s):              8
On-line CPU(s) list: 0-7
Thread(s) per core:  1
Core(s) per socket:  8
Socket(s):           1
NUMA node(s):        2
Model:               2.1 (pvr 004b 0201)
Model name:          POWER8 (architected), altivec supported
Hypervisor vendor:   KVM
Virtualization type: para
L1d cache:           64K
L1i cache:           32K
NUMA node0 CPU(s):   0,1,4-7
NUMA node1 CPU(s):   2,3


3. reboot the guest
# lscpu
Architecture:        ppc64le
Byte Order:          Little Endian
CPU(s):              8
On-line CPU(s) list: 0-7
Thread(s) per core:  1
Core(s) per socket:  8
Socket(s):           1
NUMA node(s):        4
Model:               2.1 (pvr 004b 0201)
Model name:          POWER8 (architected), altivec supported
Hypervisor vendor:   KVM
Virtualization type: para
L1d cache:           64K
L1i cache:           32K
NUMA node0 CPU(s):   0-7
NUMA node1 CPU(s):
NUMA node2 CPU(s):
NUMA node3 CPU(s):


Without this patch, Case 2 crashes the guest during hotplug, i.e
issue reported in https://github.com/linuxppc/linux/issues/184

Regards,
-Satheesh.

> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 3a048e98a132..1b2d25a3c984 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -483,6 +483,15 @@ static int numa_setup_cpu(unsigned long lcpu)
>  	if (nid < 0 || !node_possible(nid))
>  		nid = first_online_node;
> 
> +	if (NODE_DATA(nid) == NULL) {
> +		/*
> +		 * Default to using the nearest node that has memory installed.
> +		 * Otherwise, it would be necessary to patch the kernel MM code
> +		 * to deal with more memoryless-node error conditions.
> +		 */
> +		nid = first_online_node;
> +	}
> +
>  	map_cpu_to_node(lcpu, nid);
>  	of_node_put(cpu);
>  out:
> -- 
> 2.17.2
>