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. 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 The emulator thread is able to interrupt qemu vcpu0 at SCHED_NORMAL priority. With FIFO priority: Now, with qemu vcpu0 at SCHED_FIFO priority, which is necessary to avoid the following scenario: (*) 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) And the following code pattern by vcpu0: 1. submit IO. 2. busy spin. The emulator thread is unable to interrupt vcpu0 thread (vcpu0 busy spinning at SCHED_FIFO, emulator thread at SCHED_NORMAL), and you get a hang at boot as follows: 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 So to fix this problem, the patchset changes the priority of the VCPU thread (to fix (*)), only when taking spinlocks. Does that make sense now? > > > To fix this issue, only change guest vcpu-0 to FIFO priority > > on spinlock critical sections (see patch). > > This doesn't make sense. So you're saying that if you run all VCPUs as > FIFO things come apart? Why? Please see above. > And why can't they still come apart when the guest holds a spinlock? Hopefully the above makes sense.
