On 02/27/2014 08:15 PM, Paolo Bonzini wrote:
[...]

quoted

But neither of the VCPUs being kicked here are halted -- they're either
running or runnable (descheduled by the hypervisor).

/me actually looks at Waiman's code...

Right, this is really different from pvticketlocks, where the *unlock*
primitive wakes up a sleeping VCPU.  It is more similar to PLE
(pause-loop exiting).

Adding to the discussion, I see there are two possibilities here,
considering that in undercommit cases we should not exceed
HEAD_SPIN_THRESHOLD,

1. the looping vcpu in pv_head_spin_check() should do halt()
considering that we have done enough spinning (more than typical
lock-hold time), and hence we are in potential overcommit.

2. multiplex kick_cpu to do directed yield in qspinlock case.
But this may result in some ping ponging?