From: Mark Rutland <mark.rutland@arm.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Nicolas Saenz Julienne <nsaenzju@redhat.com>,
paulmck <paulmck@kernel.org>, maz <maz@kernel.org>,
frederic <frederic@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
rcu <rcu@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>,
kvmarm@lists.cs.columbia.edu,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
Anup Patel <Anup.Patel@wdc.com>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM
Date: Tue, 4 Jan 2022 16:39:19 +0000 [thread overview]
Message-ID: <YdR4N9QVYOzjowAb@FVFF77S0Q05N> (raw)
In-Reply-To: <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com>
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
I also see we'll need to add some lockdep/irq-tracing management to arm64, and
it probably makes sense to fold that into common helpers, so I'll have a play
with that tomorrow.
Thanks,
Mark.
next prev parent reply other threads:[~2022-01-04 16:39 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-17 11:51 Possible nohz-full/RCU issue in arm64 KVM Nicolas Saenz Julienne
2021-12-17 13:21 ` Mark Rutland
2021-12-17 14:15 ` Nicolas Saenz Julienne
2021-12-17 14:38 ` Mark Rutland
2021-12-17 15:54 ` Paolo Bonzini
2021-12-17 16:07 ` Paul E. McKenney
2021-12-17 16:20 ` Nicolas Saenz Julienne
2021-12-17 16:43 ` Paul E. McKenney
2021-12-17 16:34 ` Paolo Bonzini
2021-12-17 16:45 ` Paul E. McKenney
2021-12-17 17:02 ` Paolo Bonzini
2021-12-17 17:12 ` Paul E. McKenney
2021-12-17 17:23 ` Paolo Bonzini
2021-12-17 17:47 ` Paul E. McKenney
2022-01-04 16:39 ` Mark Rutland [this message]
2022-01-04 17:07 ` Paolo Bonzini
2022-01-11 11:32 ` Nicolas Saenz Julienne
2022-01-11 12:23 ` Mark Rutland
2021-12-17 14:51 ` Paolo Bonzini
2021-12-20 14:28 ` Marc Zyngier
2021-12-20 16:10 ` Frederic Weisbecker
2022-01-04 13:24 ` Mark Rutland
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