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

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Date:   Fri, 17 Dec 2021 17:34:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     paulmck@...nel.org
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 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

* or the thread will be preempted

and either will keep RCU happy as far as I understand.

Paolo