From: Paolo Bonzini <pbonzini@redhat.com>
To: paulmck@kernel.org
Cc: maz <maz@kernel.org>, frederic <frederic@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
kvmarm@lists.cs.columbia.edu, rcu <rcu@vger.kernel.org>,
Nicolas Saenz Julienne <nsaenzju@redhat.com>,
Will Deacon <will@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM
Date: Fri, 17 Dec 2021 17:34:04 +0100 [thread overview]
Message-ID: <f3d6095a-70aa-3da0-08a6-3caff2638809@redhat.com> (raw)
In-Reply-To: <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1>
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
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next prev parent reply other threads:[~2021-12-17 16:34 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 [this message]
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
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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