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

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall
Date 2017-09-22 15:10 +0200
Message-ID <usvSq-6uj-13@gated-at.bofh.it> (permalink)
References (4 earlier) <uskNj-8fY-5@gated-at.bofh.it> <ust4f-4Qp-39@gated-at.bofh.it> <usv61-5Zz-13@gated-at.bofh.it> <usvpo-65F-17@gated-at.bofh.it> <usvpo-65F-19@gated-at.bofh.it>
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On Fri, Sep 22, 2017 at 09:36:39AM -0300, Marcelo Tosatti wrote:
> On Fri, Sep 22, 2017 at 02:31:07PM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 22, 2017 at 09:16:40AM -0300, Marcelo Tosatti wrote:
> > > On Fri, Sep 22, 2017 at 12:00:05PM +0200, Peter Zijlstra wrote:
> > > > On Thu, Sep 21, 2017 at 10:10:41PM -0300, Marcelo Tosatti wrote:
> > > > > When executing guest vcpu-0 with FIFO:1 priority, which is necessary
> > > > > to
> > > > > deal with the following situation:
> > > > > 
> > > > > VCPU-0 (housekeeping VCPU)              VCPU-1 (realtime VCPU)
> > > > > 
> > > > > raw_spin_lock(A)
> > > > > interrupted, schedule task T-1          raw_spin_lock(A) (spin)
> > > > > 
> > > > > raw_spin_unlock(A)
> > > > > 
> > > > > Certain operations must interrupt guest vcpu-0 (see trace below).
> > > > 
> > > > Those traces don't make any sense. All they include is kvm_exit and you
> > > > can't tell anything from that.
> > > 
> > > Hi Peter,
> > > 
> > > OK lets describe whats happening:
> > > 
> > > With QEMU emulator thread and vcpu-0 sharing a physical CPU
> > > (which is a request from several NFV customers, to improve
> > > guest packing), the following occurs when the guest generates 
> > > the following pattern:
> > > 
> > > 		1. submit IO.
> > > 		2. busy spin.
> > 
> > User-space spinning is a bad idea in general and terminally broken in
> > a RT setup. Sounds like you need to go fix qemu to not suck.
> 
> One can run whatever application they want on the housekeeping
> vcpus. This is why rteval exists.

Nobody cares about other tasks. The problem is between the VCPU and
emulator thread. They get a priority inversion and live-lock because of
spin-waiting.

> This is not the realtime vcpu we are talking about.

You're being confused, its a RT _guest_, all VCPUs _must_ be RT.
Because, as you ran into, the guest functions as a whole, not as a bunch
of individual CPUs.

> We can fix the BIOS, which is hanging now, but userspace can 
> do whatever it wants, on non realtime vcpus (again, this is why
> rteval test exists and is used by the -RT community as 
> a testcase).

But nobody cares what other tasks on the system do, all you care about
is that the VCPUs make deterministic forward progress.

> I haven't understood what is the wrong with the patch? Are you trying
> to avoid pollution of the spinlock codepath to keep it simple?

Your patch is voodoo programming. You don't solve the actual problem,
you try and paper over it.

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Thread

[patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-21 13:50 +0200
  Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2017-09-21 15:40 +0200
    Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-21 16:10 +0200
      Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 03:20 +0200
        Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 12:10 +0200
          Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 13:00 +0200
            Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-23 13:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-23 15:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-24 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 05:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-25 11:20 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-25 17:20 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2017-09-25 18:30 +0200
          Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:20 +0200
            Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 14:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:40 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 04:30 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-25 10:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 04:30 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Peter Zijlstra <peterz@infradead.org> - 2017-09-25 11:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Thomas Gleixner <tglx@linutronix.de> - 2017-09-25 12:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Jan Kiszka <jan.kiszka@siemens.com> - 2017-09-25 20:40 +0200

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