linux-kernel - RE: [????] RE: [PATCH RESEND^2] x86/paravirt: add backoff mechanism to virt_spin

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Message-ID: <bb474c693d77428eb0336566150a1ea3@baidu.com>
Date: Thu, 14 Aug 2025 03:10:46 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: "Guo, Wangyang" <wangyang.guo@...el.com>, Peter Zijlstra
	<peterz@...radead.org>
CC: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Borislav
 Petkov" <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li, Tianyou"
	<tianyou.li@...el.com>, Tim Chen <tim.c.chen@...ux.intel.com>
Subject: RE: [????] RE: [PATCH RESEND^2] x86/paravirt: add backoff mechanism
 to virt_spin_lock

> 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@baidu.com/


Furthermore, I think the virt_spin_lock needs to be optimized.

Br
-Li

> 2. the critical section is very short; spin wait is faster than pv_kick.
> 
> BR
> Wangyang