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

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Message-ID: <20250814103953.GU4067720@noisy.programming.kicks-ass.net>
Date: Thu, 14 Aug 2025 12:39:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Guo, Wangyang" <wangyang.guo@...el.com>
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 Thu, Aug 14, 2025 at 01:26:59AM +0000, Guo, Wangyang 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.

In the non-overcommit, physically pinned case, there is a knob to use
native spinlocks.