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@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. 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
