linux-kernel - Re: [PATCH v3 0/7] mm/kvm: Improve parallelism for access bit harvesting

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Date: Fri, 12 Apr 2024 11:41:11 -0700
From: David Matlack <dmatlack@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>, Yu Zhao <yuzhao@...gle.com>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Sean Christopherson <seanjc@...gle.com>,
	Jonathan Corbet <corbet@....net>, James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Shaoqin Huang <shahuang@...hat.com>, Gavin Shan <gshan@...hat.com>,
	Ricardo Koller <ricarkol@...gle.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	David Rientjes <rientjes@...gle.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/7] mm/kvm: Improve parallelism for access bit
 harvesting

On 2024-04-01 11:29 PM, James Houghton wrote:
> This patchset adds a fast path in KVM to test and clear access bits on
> sptes without taking the mmu_lock. It also adds support for using a
> bitmap to (1) test the access bits for many sptes in a single call to
> mmu_notifier_test_young, and to (2) clear the access bits for many ptes
> in a single call to mmu_notifier_clear_young.

How much improvement would we get if we _just_ made test/clear_young
lockless on x86 and hold the read-lock on arm64? And then how much
benefit does the bitmap look-around add on top of that?