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

[Jan Kiszka]

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@xxxxxxxxxx>
> 

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.

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

Jan

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