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

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Date:   Fri, 17 Dec 2021 17:20:21 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     paulmck@...nel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Mark Rutland <mark.rutland@....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

Hi Paul,

On Fri, 2021-12-17 at 08:07 -0800, Paul E. McKenney wrote:
> 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.

Are you referring to the user_mode() check in irqentry_enter()? If so I don't
think it'll help, arm64 doesn't use that function. It directly calls
enter_from_{user,kernel}_mode() through its custom entry/exit routines.

Regards,

-- 
Nicolás Sáenz