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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 17 Jun 2020 13:04:52 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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@de.ibm.com>
> >> ---
> >>  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@redhat.com>  
> 
> 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.
> 

Not sure if I would count this as a real bug -- it's mostly just that a
large enough memory allocation may fail or draw the wrath of the oom
killer. It still sucks; but I'm wondering why we trigger this after
seven years.