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Date: Wed, 13 Aug 2025 16:33:40 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wangyang Guo <wangyang.guo@intel.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
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	kvm@vger.kernel.org, Tianyou Li <tianyou.li@intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH RESEND^2] x86/paravirt: add backoff mechanism to
 virt_spin_lock
Message-ID: <20250813143340.GN4067720@noisy.programming.kicks-ass.net>
References: <20250813005043.1528541-1-wangyang.guo@intel.com>
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In-Reply-To: <20250813005043.1528541-1-wangyang.guo@intel.com>

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.