Re: Timer delays in VM

[Peter Maydell]

.

On Mon, 28 Feb 2022 at 07:04, Eugene Huang <eugeneh@nvidia.com> wrote:
> I am running qemu on an arm64 CentOS host. Inside a ubuntu VM, a process runs 
> a timer created using timer_t:
>
> ev.sigev_notify_function = m_callback;
>
> …
>
> timer_create(CLOCK_MONOTONIC, &ev, &m_timer_t);
>
>
>
> This timer sometimes has significant delays. For example, the 50 ms timer can 
> have a callback delay of 100ms.
>
>
>
> I did a host kernel trace and see a lot of WFx kvm_exits, and  the following 
> events between kvm_exit and kvm_entry:

For questions that are really about KVM and not QEMU, you'll probably
find more experts on kvmarm@lists.cs.columbia.edu. My non-expert
look at this series of events:

> kvm_exit
> kvm_wfx_arm64

The guest kernel has nothing to do, so it idles,
waiting for the next interrupt.

> kvm_get_timer_map
> sched_switch
> kvm_timer_save_state

So the host schedules something else instead of this vCPU
(I think).

> kvm_timer_update_irq
> vgic_update_irq_pending

At some later date, the timer IRQ fires...

> kvm_timer_restore_state
> kvm_vcpu_wakeup
> kvm_arm_setup_debug
> kvm_arm_set_dreg32
> kvm_entry

...so we reschedule the vCPU and re-enter the guest.

I don't know why the timer might have significant delays,
but of course if your host system is very heavily loaded we
might not get round to rescheduling the vCPU for a bit.

thanks
-- PMM