On Fri, Sep 22, 2017 at 09:36:39AM -0300, Marcelo Tosatti wrote: > 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.
Nobody cares about other tasks. The problem is between the VCPU and emulator thread. They get a priority inversion and live-lock because of spin-waiting. > This is not the realtime vcpu we are talking about. You're being confused, its a RT _guest_, all VCPUs _must_ be RT. Because, as you ran into, the guest functions as a whole, not as a bunch of individual CPUs. > 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). But nobody cares what other tasks on the system do, all you care about is that the VCPUs make deterministic forward progress. > 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? Your patch is voodoo programming. You don't solve the actual problem, you try and paper over it.
