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

From: David Matlack <dmatlack@google.com>
To: James Houghton <jthoughton@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>, Yu Zhao <yuzhao@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Sean Christopherson <seanjc@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Shaoqin Huang <shahuang@redhat.com>,
	Gavin Shan <gshan@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	David Rientjes <rientjes@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, linux-mm@kvack.org,
	linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/7] mm/kvm: Improve parallelism for access bit harvesting
Date: Fri, 12 Apr 2024 11:41:11 -0700	[thread overview]
Message-ID: <ZhmAR1akBHjvZ9_4@google.com> (raw)
In-Reply-To: <20240401232946.1837665-1-jthoughton@google.com>

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?