Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks

[Attilio Rao <attilio.rao@citrix.com>]

On 21/03/12 14:25, Stephan Diestelhorst wrote:
> On Wednesday 21 March 2012, 13:49:28 Attilio Rao wrote:
>    
>> On 21/03/12 13:22, Stephan Diestelhorst wrote:
>>      
>>> On Wednesday 21 March 2012, 13:04:25 Attilio Rao wrote:
>>>
>>>        
>>>> On 21/03/12 10:20, Raghavendra K T wrote:
>>>>
>>>>          
>>>>> From: Jeremy Fitzhardinge<jeremy.fitzhardinge@citrix.com>
>>>>>
>>>>> Rather than outright replacing the entire spinlock implementation in
>>>>> order to paravirtualize it, keep the ticket lock implementation but add
>>>>> a couple of pvops hooks on the slow patch (long spin on lock, unlocking
>>>>> a contended lock).
>>>>>
>>>>> Ticket locks have a number of nice properties, but they also have some
>>>>> surprising behaviours in virtual environments.  They enforce a strict
>>>>> FIFO ordering on cpus trying to take a lock; however, if the hypervisor
>>>>> scheduler does not schedule the cpus in the correct order, the system can
>>>>> waste a huge amount of time spinning until the next cpu can take the lock.
>>>>>
>>>>> (See Thomas Friebel's talk "Prevent Guests from Spinning Around"
>>>>> http://www.xen.org/files/xensummitboston08/LHP.pdf  for more details.)
>>>>>
>>>>> To address this, we add two hooks:
>>>>>     - __ticket_spin_lock which is called after the cpu has been
>>>>>       spinning on the lock for a significant number of iterations but has
>>>>>       failed to take the lock (presumably because the cpu holding the lock
>>>>>       has been descheduled).  The lock_spinning pvop is expected to block
>>>>>       the cpu until it has been kicked by the current lock holder.
>>>>>     - __ticket_spin_unlock, which on releasing a contended lock
>>>>>       (there are more cpus with tail tickets), it looks to see if the next
>>>>>       cpu is blocked and wakes it if so.
>>>>>
>>>>> When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub
>>>>> functions causes all the extra code to go away.
>>>>>
>>>>>
>>>>>            
>>>> I've made some real world benchmarks based on this serie of patches
>>>> applied on top of a vanilla Linux-3.3-rc6 (commit
>>>> 4704fe65e55fb088fbcb1dc0b15ff7cc8bff3685), with both
>>>> CONFIG_PARAVIRT_SPINLOCK=y and n, which means essentially 4 versions
>>>> compared:
>>>> * vanilla - CONFIG_PARAVIRT_SPINLOCK - patch
>>>> * vanilla + CONFIG_PARAVIRT_SPINLOCK - patch
>>>> * vanilla - CONFIG_PARAVIRT_SPINLOCK + patch
>>>> * vanilla + CONFIG_PARAVIRT_SPINLOCK + patch
>>>>
>>>>
>>>>          
>>> [...]
>>>
>>>        
>>>> == Results
>>>> This test points in the direction that Jeremy's rebased patches don't
>>>> introduce a peformance penalty at all, but also that we could likely
>>>> consider CONFIG_PARAVIRT_SPINLOCK option removal, or turn it on by
>>>> default and suggest disabling just on very old CPUs (assuming a
>>>> performance regression can be proven there).
>>>>
>>>>          
>>> Very interesting results, in particular knowing that in the one guest
>>> case things do not get (significantly) slower due to the added logic
>>> and LOCKed RMW in the unlock path.
>>>
>>> AFAICR, the problem really became apparent when running multiple guests
>>> time sharing the physical CPUs, i.e., two guests with eight vCPUs each
>>> on an eight core machine.  Did you look at this setup with your tests?
>>>
>>>
>>>        
>> Please note that my tests are made on native Linux, without XEN involvement.
>>
>> You maybe meant that the spinlock paravirtualization became generally
>> useful in the case you mentioned? (2 guests, 8vpcu + 8vcpu)?
>>      
> Yes, that is what I meant.  Just to clarify why you do not see any
> speed-ups, and were wondering why.  If the whole point of the exercise
> was to see that there are no perforamnce regressions, fine.  In that
> case I misunderstood.
>    

Yes, that's right, I just wanted to measure (possible) overhead in 
native Linux and the cost of leaving CONFIG_PARAVIRT_SPINLOCK on.

Thanks,
Attilio