linux-kernel - Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT

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Date:   Fri, 22 Sep 2017 09:36:39 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, mingo@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO
 hypercall

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.

This is not the realtime vcpu we are talking about.

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).

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?