From: Peter Maydell <peter.maydell@linaro.org>
To: Eugene Huang <eugeneh@nvidia.com>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: Timer delays in VM
Date: Mon, 28 Feb 2022 11:02:24 +0000 [thread overview]
Message-ID: <CAFEAcA9+cof3gHwTEht4j813hoTsSdstHd568BZEd5fm8n+n2w@mail.gmail.com> (raw)
In-Reply-To: <BYAPR12MB3192248D73D9C8AF64076A62D9019@BYAPR12MB3192.namprd12.prod.outlook.com>
.
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
next prev parent reply other threads:[~2022-02-28 11:04 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-28 6:22 Timer delays in VM Eugene Huang
2022-02-28 11:02 ` Peter Maydell [this message]
2022-02-28 18:02 Eugene Huang
2022-02-28 21:02 ` Marc Zyngier
2022-03-01 9:06 ` Andrew Jones
2022-03-01 19:03 ` Eugene Huang
2022-03-02 2:27 ` Eugene Huang
2022-03-02 7:28 ` Marc Zyngier
2022-03-03 5:49 ` Eugene Huang
2022-03-03 14:42 ` Marc Zyngier
2022-03-08 7:50 ` Eugene Huang
2022-03-08 9:34 ` Marc Zyngier
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