Re: [Xen-devel] [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 04/03/14 15:15, Waiman Long wrote:
> On 03/03/2014 05:55 AM, Paolo Bonzini wrote:
>> Il 28/02/2014 18:06, Waiman Long ha scritto:
>>> On 02/26/2014 12:07 PM, Konrad Rzeszutek Wilk wrote:
>>>> On Wed, Feb 26, 2014 at 10:14:24AM -0500, Waiman Long wrote:
>>>>> Locking is always an issue in a virtualized environment as the virtual
>>>>> CPU that is waiting on a lock may get scheduled out and hence block
>>>>> any progress in lock acquisition even when the lock has been freed.
>>>>>
>>>>> One solution to this problem is to allow unfair lock in a
>>>>> para-virtualized environment. In this case, a new lock acquirer can
>>>>> come and steal the lock if the next-in-line CPU to get the lock is
>>>>> scheduled out. Unfair lock in a native environment is generally not a
>>>> Hmm, how do you know if the 'next-in-line CPU' is scheduled out? As
>>>> in the hypervisor knows - but you as a guest might have no idea
>>>> of it.
>>>
>>> I use a heart-beat counter to see if the other side responses within a
>>> certain time limit. If not, I assume it has been scheduled out probably
>>> due to PLE.
>>
>> PLE is unnecessary if you have "true" pv spinlocks where the
>> next-in-line schedules itself out with a hypercall (Xen) or hlt
>> instruction (KVM).  Set a bit in the qspinlock before going to sleep,
>> and the lock owner will know that it needs to kick the next-in-line.
>>
>> I think there is no need for the unfair lock bits.  1-2% is a pretty
>> large hit.
>>
>> Paolo
> 
> I don't think that PLE is something that can be controlled by software.

You can avoid PLE by not issuing PAUSE instructions when spinning.  You
may want to consider this if you have a lock that explicitly deschedules
the VCPU while waiting (or just deschedule before PLE would trigger).

> It is done in hardware. I maybe wrong. Anyway, I plan to add code to
> schedule out the CPUs waiting in the queue except the first 2 in the
> next version of the patch.

I think you should deschedule all waiters.

> The PV code in the v5 patch did seem to improve benchmark performance
> with moderate to heavy spinlock contention.

The goal of PV aware locks is to improve performance when locks are
contented /and/ VCPUs are over-committed.  Is this something you're
actually measuring?  It's not clear to me.

David

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