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

[Peter Zijlstra]

On Thu, Aug 14, 2025 at 03:10:46AM +0000, Li,Rongqing 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.
> 
> If vm is not overcommit, guest should have KVM_HINTS_REALTIME, I think native qspinlock should be better
> Could you try test this patch
> https://patchwork.kernel.org/project/kvm/patch/20250722110005.4988-1-lirongqing@xxxxxxxxx/

Right, that's the knob.

> Furthermore, I think the virt_spin_lock needs to be optimized.

Why would virt_spin_lock() need to be optimized? It is the fallback
case; but it is terrible in all possible ways.