Re: [patch 0/3] KVM KVM_HC_RT_PRIO hypercall support

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Sep 21, 2017 at 07:45:32PM +0200, Jan Kiszka wrote:
> On 2017-09-21 13:38, 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).
> > 
> > To fix this issue, only change guest vcpu-0 to FIFO priority
> > on spinlock critical sections (see patch).
> > 
> > Hang trace
> > ==========
> > 
> > Without FIFO priority:
> > 
> > qemu-kvm-6705  [002] ....1.. 767785.648964: kvm_exit: reason IO_INSTRUCTION rip 0xe8fe info 1f00039 0
> > qemu-kvm-6705  [002] ....1.. 767785.648965: kvm_exit: reason IO_INSTRUCTION rip 0xe911 info 3f60008 0
> > qemu-kvm-6705  [002] ....1.. 767785.648968: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
> > qemu-kvm-6705  [002] ....1.. 767785.648971: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
> > qemu-kvm-6705  [002] ....1.. 767785.648974: kvm_exit: reason IO_INSTRUCTION rip 0xb514 info 3f60000 0
> > qemu-kvm-6705  [002] ....1.. 767785.648977: kvm_exit: reason PENDING_INTERRUPT rip 0x8052 info 0 0
> > qemu-kvm-6705  [002] ....1.. 767785.648980: kvm_exit: reason IO_INSTRUCTION rip 0xeee6 info 200040 0
> > qemu-kvm-6705  [002] ....1.. 767785.648999: kvm_exit: reason EPT_MISCONFIG rip 0x2120 info 0 0
> > 
> > With FIFO priority:
> > 
> > qemu-kvm-7636  [002] ....1.. 768218.205065: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
> > qemu-kvm-7636  [002] ....1.. 768218.205068: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
> > qemu-kvm-7636  [002] ....1.. 768218.205071: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
> > qemu-kvm-7636  [002] ....1.. 768218.205074: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
> > qemu-kvm-7636  [002] ....1.. 768218.205077: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
> > ..
> > 
> > Performance numbers (kernel compilation with make -j2)
> > ======================================================
> > 
> > With hypercall: 4:40.  (make -j2)
> > Without hypercall: 3:38.  (make -j2)
> > 
> > Note for NFV workloads spinlock performance is not relevant
> > since DPDK should not enter the kernel (and housekeeping vcpu
> > performance is far from a key factor).
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> 
> That sounds familiar, though not yet the same: :)
> 
> http://git.kiszka.org/?p=linux-kvm.git;a=shortlog;h=refs/heads/queues/paravirt-sched
> (paper: http://lwn.net/images/conf/rtlws11/papers/proc/p18.pdf)
> 
> I suppose your goal is not to enable the host to follow the guest
> scheduler priority completely but only to have prio-ceiling for such
> short critical sections. Maybe still useful to think ahead about future
> extensions when actually introducing such an interface.

Hi Jan!

Hum... I'll take a look at your interface/paper and get back to you.

> But shouldn't there be some limits on the maximum prio the guest can select?

The SCHED_FIFO prio is fixed, selectable when QEMU starts. Do you
envision any other use case than a fixed priority value selectable
at QEMU initialization?

Thanks