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.
