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([2001:b07:6468:f312:48f9:bea:a04c:3dfe]) by smtp.googlemail.com with ESMTPSA id h204sm7828484wmh.33.2021.12.17.08.34.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Dec 2021 08:34:08 -0800 (PST) Message-ID: Date: Fri, 17 Dec 2021 17:34:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: Possible nohz-full/RCU issue in arm64 KVM To: paulmck@kernel.org Cc: Mark Rutland , Nicolas Saenz Julienne , maz , frederic , linux-kernel , rcu , Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel References: <70f112072d9496d21901946ea82832d3ed3a8cb2.camel@redhat.com> <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1> From: Paolo Bonzini In-Reply-To: <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20211217_083413_601173_91780C08 X-CRM114-Status: GOOD ( 12.42 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 12/17/21 17:07, Paul E. McKenney wrote: >> rcu_note_context_switch() is a point-in-time notification; it's not strictly >> necessary, but it may improve performance a bit by avoiding unnecessary IPIs >> from the RCU subsystem. >> >> There's no benefit from doing it when you're back from the guest, because at >> that point the CPU is just running normal kernel code. > > Do scheduling-clock interrupts from guest mode have the "user" parameter > set? If so, that would keep RCU happy. No, thread is in supervisor mode. But after every interrupt (timer tick or anything), one of three things can happen: * KVM will go around the execution loop and invoke rcu_note_context_switch() again * or KVM will go back to user space * or the thread will be preempted and either will keep RCU happy as far as I understand. Paolo _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel