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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Sep 29, 2017 at 07:05:41PM +0200, Paolo Bonzini wrote:
> On 29/09/2017 18:40, Marcelo Tosatti wrote:
> >> If you know you have this kind disk workload, you must use virtio-blk or
> >> virtio-scsi with iothreads and place the iothreads on their own physical
> >> CPUs.
> >>
> >> Among "run arbitrary workloads", "run real-time workloads", "pack stuff
> >> into as few physical CPUs as possible", you can only pick two.
> >
> > 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.

>  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] http://static.lwn.net/images/conf/rtlws11/papers/proc/p18.pdf which
> also mentions pv scheduling