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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 17 Jun 2020 10:36:20 +0200
Christian Borntraeger <borntraeger@de.ibm.com> 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.

Right, I added that #define when I was not sure yet whether we would
need to support chip/pin routing, and it just was never revisited
again. I think we can safely assume that it is uninteresting on s390.
Getting it down to the minimum seems to be the right thing to do.

> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  arch/s390/include/asm/kvm_host.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>