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

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Message-ID: <20211217164535.GU641268@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 17 Dec 2021 08:45:35 -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 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?

> * 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.  ;-)

						Thanx, Paul