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Message-ID: <a0ceaa0f-2a05-0db6-2a94-45d150758c34@redhat.com>
Date: Wed, 21 Nov 2018 20:06:47 +0100
From: Laurent Vivier <lvivier@...hat.com>
To: Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>,
Satheesh Rajendran <satheera@...ibm.com>,
linux-kernel@...r.kernel.org,
Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/numa: fix hot-added CPU on memory-less node
On 15/11/2018 10:19, Satheesh Rajendran wrote:
> 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@...hat.com>
>> ---
>> 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
>>
>
I have worked a while on this problem, and I don't see any easy fix for
that. It seems kernel is not ready to online a memory-less/cpu-less node
when someone hotplug a CPU in it. I think we have to fix several areas
to be able to do that.
Perhaps someone from IBM could have a better view on what we need?
Michael? Nathan?
Thanks,
Laurent
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