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

[Waiman Long <waiman.long@hp.com>]

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
> 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?
>
>
>

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.

-Longman