Date: Fri, 6 Feb 2015 14:46:35 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: riel@redhat.com
Cc: kvm@vger.kernel.org, borntraeger@de.ibm.com, linux-kernel@vger.kernel.org,
        mtosatti@redhat.com, mingo@kernel.orgm, ak@linux.intel.com,
        oleg@redhat.com, masami.hiramatsu.pt@hitachi.com,
        paulmck@linux.vnet.ibm.com, lcapitulino@redhat.com,
        pbonzini@redhat.com
Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state
 when running KVM guest
Message-ID: <20150206134631.GA21905@lerouge>
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On Thu, Feb 05, 2015 at 03:23:47PM -0500, riel@redhat.com wrote:
> When running a KVM guest on a system with NOHZ_FULL enabled

I just need to clarify the motivation first, does the above situation
really happen? Ok some distros enable NOHZ_FULL to let the user stop
the tick in userspace. So most of the time, CONFIG_NOHZ_FULL=y but
nohz full is runtime disabled (we need to pass a nohz_full= boot
parameter to enable it). And when it is runtime disabled, there should
be no rcu nocb CPU.

(Although not setting CPUs in nocb mode when nohz full is runtime disabled
is perhaps a recent change.)

So for the problem to arise, one need to enable nohz_full and run KVM
guest. And I never heard about such workloads. That said it's potentially
interesting to turn off the tick on the host when the guest runs.

>, and the
> KVM guest running with idle=poll mode, we still get wakeups of the
> rcuos/N threads.

So we need nohz_full on the host and idle=poll mode on the guest. Is it
likely to happen? (sorry, again I'm just trying to make sure we agree on
why we do this change).

> 
> This problem has already been solved for user space by telling the
> RCU subsystem that the CPU is in an extended quiescent state while
> running user space code.
> 
> This patch series extends that code a little bit to make it usable
> to track KVM guest space, too.
> 
> I tested the code by booting a KVM guest with idle=poll, on a system
> with NOHZ_FULL enabled on most CPUs, and a VCPU thread bound to a
> CPU. In a 10 second interval, rcuos/N threads on other CPUs got woken
> up several times, while the rcuos thread on the CPU running the bound
> and alwasy running VCPU thread never got woken up once.

So what you're describing is to set RCU in extended quiescent state, right?
This doesn't include stopping the tick while running in guest mode? Those
are indeed two different thing, although stopping the tick most often requires
to set RCU in extended quiescent state.

> 
> Thanks to Christian Borntraeger and Paul McKenney for reviewing the
> first version of this patch series, and helping optimize patch 4/5.
> 
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