RE: [PATCH RESEND^2] x86/paravirt: add backoff mechanism to virt_spin_lock

[Guo, Wangyang]

On 8/13/2025 10:33 PM, Peter Zijlstra wrote:
> On Wed, Aug 13, 2025 at 08:50:43AM +0800, Wangyang Guo wrote:
>> When multiple threads waiting for lock at the same time, once lock owner
>> releases the lock, waiters will see lock available and all try to lock,
>> which may cause an expensive CAS storm.
>>
>> Binary exponential backoff is introduced. As try-lock attempt increases,
>> there is more likely that a larger number threads compete for the same
>> lock, so increase wait time in exponential.
> 
> You shouldn't be using virt_spin_lock() to begin with. That means you've
> misconfigured your guest.
> 
> We have paravirt spinlocks for a reason.

We have tried PARAVIRT_SPINLOCKS, it can help to reduce the contention cycles, but the throughput is not good. I think there are two factors:

1. the VM is not overcommit, each thread has its CPU resources to doing spin wait.
2. the critical section is very short; spin wait is faster than pv_kick.

BR
Wangyang