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Sat, 24 Oct 2026 14:23:12 -0700 (PDT) Received: from ASHA ([2001:630:3c1:164:1a66:daff:fe85:4026]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a359f3e00asm147825395e9.0.2026.10.24.14.23.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 Oct 2026 14:23:11 -0700 (PDT) From: Amir Noohi To: Marc Zyngier Cc: Thomas Gleixner , Oliver Upton , Radu Rendec , Igor Velkov , Fuad Tabba , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Mark Rutland , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/2] genirq, KVM: arm64: Avoid the irq descriptor lock on vCPU load Date: Sat, 24 Oct 2026 21:23:04 +0000 Message-ID: <20261024212304.2620845-1-amirmnoohi@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <87tsmb8gdv.wl-maz@kernel.org> References: <20261024022450.629980-1-amirmnoohi@gmail.com> <87tsmb8gdv.wl-maz@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261024_142315_517422_46CBFC53 X-CRM114-Status: GOOD ( 11.86 ) X-Spam-Score: -2.1 (--) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Hi Marc, On Sat, 24 Oct 2026 12:52:44 +0100, Marc Zyngier wrote: > Looking at the test case, this really isn't a representative or > realistic workload. This is a contrived setup that has trapping to > userspace as its only purpose. > > I'm not saying that this is useless observation. But it is not > something that represents something exacerbated by "real" VMs. Fair enough. I checked with real Linux guests running hackbench on the same Altra (kvmtool, in-kernel GICv3). With one vCPU per CPU, and in lighter hackbench runs, it makes no measurable difference, but it does once busy vCPUs share CPUs. With four 31-vCPU VMs on 31 CPUs (124 vCPUs), all running "hackbench -g 31 -f 20 -l 2000", each VM takes 76.5s on mainline and 60.4s with the series, so 21% less time (8 runs per kernel, no overlap). In a profiled run irq_set_irqchip_state() was called ~2.4M times/s and 47% of those calls hit contention. With the series the contention on that path drops to ~7 per second. So the case that matters is overcommitted hosts with busy vCPUs. I'll make that the main justification in v2 instead of the exit-to-userspace test. Thanks, Amir