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

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Message-ID: <4c369ca2-2ad0-4a02-90e9-2d82df6c289d@redhat.com>
Date:   Fri, 17 Dec 2021 18:02:23 +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 17:45, 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.

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.  ;-)