Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall\

From: Paolo Bonzini
Date: Mon Oct 02 2017 - 08:30:44 EST

On 29/09/2017 22:17, Marcelo Tosatti wrote:
> On Fri, Sep 29, 2017 at 07:05:41PM +0200, Paolo Bonzini wrote:
>> On 29/09/2017 18:40, Marcelo Tosatti wrote:
>>> Thats not the state of things (userspace in vcpu-0 is not specially tailored
>>> to not violate latencies in vcpu-1): that is not all user triggered
>>> actions can be verified.
>>> Think "updatedb", and so on...
>> _Which_ spinlock is it that can cause unwanted latency while running
>> updatedb on VCPU0 and a real-time workload on VCPU1, and only so on virt
>> because of the emulator thread?
> Hundreds of them (the one being hit is in timer_interrupt), but i went
> to check and there are hundreds of raw spinlocks shared between the
> kernel threads that run on isolated CPUs and vcpu-0.
>> Is this still broken if you set up
>> priorities for the emulator thread correctly and use PI mutexes in QEMU?
> I don't see why it would not, if you have to schedule the emulator
> thread to process and inject I/O interrupts for example.

Yes, you're right if it's interrupt injections. If it's unexpected disk
accesses, you can just add a QEMU I/O thread on a different physical
CPU. The same physical CPU can host I/O threads for different guests if
you expect them to do little.

I don't understand why is it correct to delay interrupt injection just
because VCPU0 is running in a spinlock-protected region? I just cannot
see the reason why it's safe and not a recipe for priority inversions.


>> And if so, what is the cause of interruptions in the emulator thread
>> and how are these interruptions causing the jitter?
> Interrupt injections.
>> Priorities and priority inheritance (or lack of them) is a _known_
>> issue. Jan was doing his KVM-RT things in 2009 and he was talking about
>> priorities[1] back then. The effect of correct priorities is to _lower_
>> jitter, not to make it worse, and anyway certainly not worse than
>> SCHED_NORMAL I/O thread. Once that's fixed, we can look at other problems.
>> Paolo
>> [1] which
>> also mentions pv scheduling