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

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Message-ID: <20211217171218.GV641268@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 17 Dec 2021 09:12:18 -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 06:02:23PM +0100, Paolo Bonzini wrote:
> On 12/17/21 17:45, Paul E. McKenney wrote:
> > 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?
> 
> Yes, that code runs from ioctl(KVM_RUN) and the ioctl will return to the
> calling process.

Intriguing.  A user process within the guest OS or a user process outside
of any guest OS, that is, within the host?

							Thanx, Paul

> Paolo
> 
> > > * 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.  ;-)
>