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

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Message-ID: <20250814104156.GV4067720@noisy.programming.kicks-ass.net>
Date: Thu, 14 Aug 2025 12:41:56 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Li,Rongqing" <lirongqing@...du.com>
Cc: "Guo, Wangyang" <wangyang.guo@...el.com>,
	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 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@baidu.com/

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.