linux-kernel - Re: Possible nohz-full/RCU issue in arm64 KVM

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Message-ID: <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 17 Dec 2021 08:07:49 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        maz <maz@...nel.org>, frederic <frederic@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rcu <rcu@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM

On Fri, Dec 17, 2021 at 04:54:22PM +0100, Paolo Bonzini wrote:
> On 12/17/21 15:38, Mark Rutland wrote:
> > For example kvm_guest_enter_irqoff() calls guest_enter_irq_off() which calls
> > vtime_account_guest_enter(), but kvm_guest_exit_irqoff() doesn't call
> > guest_exit_irq_off() and the call to vtime_account_guest_exit() is open-coded
> > elsewhere. Also, guest_enter_irq_off() conditionally calls
> > rcu_virt_note_context_switch(), but I can't immediately spot anything on the
> > exit side that corresponded with that, which looks suspicious.
> 
> 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.

							Thanx, Paul