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([2001:b07:6468:f312::ac3]) by smtp.googlemail.com with ESMTPSA id u14sm8273562wrf.39.2021.12.17.09.02.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Dec 2021 09:02:30 -0800 (PST) Message-ID: <4c369ca2-2ad0-4a02-90e9-2d82df6c289d@redhat.com> Date: Fri, 17 Dec 2021 18:02:23 +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 Content-Language: en-US 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> <20211217164535.GU641268@paulmck-ThinkPad-P17-Gen-1> From: Paolo Bonzini In-Reply-To: <20211217164535.GU641268@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/17/21 17:45, Paul E. McKenney wrote: > On Fri, Dec 17, 2021 at 05:34:04PM +0100, Paolo Bonzini wrote: >> 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 > > Here "user space" is a user process as opposed to a guest OS? Yes, that code runs from ioctl(KVM_RUN) and the ioctl will return to the calling process. Paolo >> * or the thread will be preempted >> >> and either will keep RCU happy as far as I understand. > > Regardless of the answer to my question above, yes, these will keep > RCU happy. ;-)