Re: [PATCH] x86/paravirt: Add kernel parameter to choose paravirt lock type

[Waiman Long <longman@redhat.com>]

On 11/01/2017 11:51 AM, Juergen Gross wrote:
> On 01/11/17 16:32, Waiman Long wrote:
>> Currently, there are 3 different lock types that can be chosen for
>> the x86 architecture:
>>
>>  - qspinlock
>>  - pvqspinlock
>>  - unfair lock
>>
>> One of the above lock types will be chosen at boot time depending on
>> a number of different factors.
>>
>> Ideally, the hypervisors should be able to pick the best performing
>> lock type for the current VM configuration. That is not currently
>> the case as the performance of each lock type are affected by many
>> different factors like the number of vCPUs in the VM, the amount vCPU
>> overcommitment, the CPU type and so on.
>>
>> Generally speaking, unfair lock performs well for VMs with a small
>> number of vCPUs. Native qspinlock may perform better than pvqspinlock
>> if there is vCPU pinning and there is no vCPU over-commitment.
>>
>> This patch adds a new kernel parameter to allow administrator to
>> choose the paravirt spinlock type to be used. VM administrators can
>> experiment with the different lock types and choose one that can best
>> suit their need, if they want to. Hypervisor developers can also use
>> that to experiment with different lock types so that they can come
>> up with a better algorithm to pick the best lock type.
>>
>> The hypervisor paravirt spinlock code will override this new parameter
>> in determining if pvqspinlock should be used. The parameter, however,
>> will override Xen's xen_nopvspin in term of disabling unfair lock.
> Hmm, I'm not sure we need pvlock_type _and_ xen_nopvspin. What do others
> think?

I don't think we need xen_nopvspin, but I don't want to remove that
without agreement from the community.
>>  DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
>>  
>>  void __init native_pv_lock_init(void)
>>  {
>> -	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>> +	if (pv_spinlock_type == locktype_unfair)
>> +		return;
>> +
>> +	if (!static_cpu_has(X86_FEATURE_HYPERVISOR) ||
>> +	   (pv_spinlock_type != locktype_auto))
>>  		static_branch_disable(&virt_spin_lock_key);
> Really? I don't think locktype_paravirt should disable the static key.

With paravirt spinlock, it doesn't matter if the static key is disabled
or not. Without CONFIG_PARAVIRT_SPINLOCKS, however, it does degenerate
into the native qspinlock. So you are right, I should check for paravirt
type as well.

Cheers,
Longman