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

[Peter Zijlstra]

On Thu, Aug 14, 2025 at 01:26:59AM +0000, Guo, Wangyang wrote:
> 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.

In the non-overcommit, physically pinned case, there is a knob to use
native spinlocks.