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

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Message-ID: <DS0PR11MB8018B027AA0738EB8B6CD55D9235A@DS0PR11MB8018.namprd11.prod.outlook.com>
Date: Thu, 14 Aug 2025 01:26:59 +0000
From: "Guo, Wangyang" <wangyang.guo@...el.com>
To: 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: [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.
2. the critical section is very short; spin wait is faster than pv_kick.

BR
Wangyang