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Re: [Xen-devel] [PATCH RFC v5 7/8] pvqspinlock, x86: Add qspinlock para-virtualization support

On 02/27/2014 10:22 AM, Raghavendra K T wrote:
On 02/27/2014 08:15 PM, Paolo Bonzini wrote:
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

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?

In the current code, the lock holder can't easily locate the CPU # of the queue head when in the unlock path. That is why I try to keep the queue head alive as long as possible so that it can take over when the lock is free. I am trying out new code to let the CPUs that are waiting other than the first 2 to go to halt to see if that will help the overcommit case.


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