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

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Date:   Tue, 11 Jan 2022 12:23:10 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, paulmck <paulmck@...nel.org>,
        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>,
        Anup Patel <Anup.Patel@....com>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM

On Tue, Jan 11, 2022 at 12:32:38PM +0100, Nicolas Saenz Julienne wrote:
> Hi Mark,
> 
> On Tue, 2022-01-04 at 16:39 +0000, Mark Rutland 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.
> > 
> > I see.
> > 
> > My main issue here was just that it's really difficult to see how the
> > entry/exit logic is balanced, and I reckon we can solve that by splitting
> > guest_{enter,exit}_irqoff() into helper functions to handle the vtime
> > accounting separately from the context tracking, so that arch code can do
> > something like:
> > 
> >   guest_timing_enter_irqoff();
> >   
> >   guest_eqs_enter_irqoff();
> >   < actually run vCPU here >
> >   guest_eqs_exit_irqoff();
> >   
> >   < handle pending IRQs here >
> >   
> >   guest_timing_exit_irqoff();
> > 
> > ... which I hope should work for RISC-V too.
> > 
> > I've had a go, and I've pushed out a WIP to:
> > 
> >   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/kvm/rcu
> 
> Had a look at the patches and they seeem OK to me.
> 
> Thanks!

Cool.

FWIW I have an updated version at:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=kvm/entry-rework

... which is largely the same approach, but the helpers got renamed, the
lockdep/tracing bits got fixed, and I've aligned mips, riscv, and x86 on the
same approach.

Once I get a free hour or so I intend to rebase that atop v5.16 and post that
out. I'll start a new thread with that, and rope in the relevant arch
maintainers (since e.g. I'm not sure what to do for ppc and s390).

Thanks,
Mark.

> 
> -- 
> Nicolás Sáenz
>