Re: [PATCH] KVM: s390: reduce number of IO pins to 1

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On 17.06.20 12:19, David Hildenbrand wrote:
> On 17.06.20 10:36, Christian Borntraeger wrote:
>> The current number of KVM_IRQCHIP_NUM_PINS results in an order 3
>> allocation (32kb) for each guest start/restart. This can result in OOM
>> killer activity even with free swap when the memory is fragmented
>> enough:
>>
>> kernel: qemu-system-s39 invoked oom-killer: gfp_mask=0x440dc0(GFP_KERNEL_ACCOUNT|__GFP_COMP|__GFP_ZERO), order=3, oom_score_adj=0
>> kernel: CPU: 1 PID: 357274 Comm: qemu-system-s39 Kdump: loaded Not tainted 5.4.0-29-generic #33-Ubuntu
>> kernel: Hardware name: IBM 8562 T02 Z06 (LPAR)
>> kernel: Call Trace:
>> kernel: ([<00000001f848fe2a>] show_stack+0x7a/0xc0)
>> kernel:  [<00000001f8d3437a>] dump_stack+0x8a/0xc0
>> kernel:  [<00000001f8687032>] dump_header+0x62/0x258
>> kernel:  [<00000001f8686122>] oom_kill_process+0x172/0x180
>> kernel:  [<00000001f8686abe>] out_of_memory+0xee/0x580
>> kernel:  [<00000001f86e66b8>] __alloc_pages_slowpath+0xd18/0xe90
>> kernel:  [<00000001f86e6ad4>] __alloc_pages_nodemask+0x2a4/0x320
>> kernel:  [<00000001f86b1ab4>] kmalloc_order+0x34/0xb0
>> kernel:  [<00000001f86b1b62>] kmalloc_order_trace+0x32/0xe0
>> kernel:  [<00000001f84bb806>] kvm_set_irq_routing+0xa6/0x2e0
>> kernel:  [<00000001f84c99a4>] kvm_arch_vm_ioctl+0x544/0x9e0
>> kernel:  [<00000001f84b8936>] kvm_vm_ioctl+0x396/0x760
>> kernel:  [<00000001f875df66>] do_vfs_ioctl+0x376/0x690
>> kernel:  [<00000001f875e304>] ksys_ioctl+0x84/0xb0
>> kernel:  [<00000001f875e39a>] __s390x_sys_ioctl+0x2a/0x40
>> kernel:  [<00000001f8d55424>] system_call+0xd8/0x2c8
>>
>> As far as I can tell s390x does not use the iopins as we bail our for
>> anything other than KVM_IRQ_ROUTING_S390_ADAPTER and the chip/pin is
>> only used for KVM_IRQ_ROUTING_IRQCHIP. So let us use a small number to
>> reduce the memory footprint.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> ---
>>  arch/s390/include/asm/kvm_host.h | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index cee3cb6455a2..6ea0820e7c7f 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -31,12 +31,12 @@
>>  #define KVM_USER_MEM_SLOTS 32
>>  
>>  /*
>> - * These seem to be used for allocating ->chip in the routing table,
>> - * which we don't use. 4096 is an out-of-thin-air value. If we need
>> - * to look at ->chip later on, we'll need to revisit this.
>> + * These seem to be used for allocating ->chip in the routing table, which we
>> + * don't use. 1 is as small as we can get to reduce the needed memory. If we
>> + * need to look at ->chip later on, we'll need to revisit this.
>>   */
>>  #define KVM_NR_IRQCHIPS 1
>> -#define KVM_IRQCHIP_NUM_PINS 4096
>> +#define KVM_IRQCHIP_NUM_PINS 1
>>  #define KVM_HALT_POLL_NS_DEFAULT 50000
>>  
>>  /* s390-specific vcpu->requests bit members */
>>
> 
> Guess it doesn't make sense to wrap all the "->chip" handling in a
> separate set of defines.
> 
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

I guess this is just the most simple solution. I am asking myself if I should add
cc stable of Fixes as I was able to trigger this by having several guests with a
reboot loop and several guests that trigger memory overcommitment.




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