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From: Rik van Riel <riel@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.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
Date: Fri, 06 Feb 2015 09:56:43 -0500	[thread overview]
Message-ID: <54D4D62B.1040000@redhat.com> (raw)
In-Reply-To: <20150206134631.GA21905@lerouge>

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On 02/06/2015 08:46 AM, Frederic Weisbecker wrote:
> 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).

We have users interested in doing just that, in order to run
KVM guests with the least amount of perturbation to the guest.


- -- 
All rights reversed
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  parent reply	other threads:[~2015-02-06 14:57 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05 20:23 riel
2015-02-05 20:23 ` [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user_enter/exit riel
2015-02-05 23:55   ` Paul E. McKenney
2015-02-06 10:15     ` Paolo Bonzini
2015-02-06 13:41       ` Paul E. McKenney
2015-02-06 17:22   ` Frederic Weisbecker
2015-02-06 18:20     ` Rik van Riel
2015-02-06 18:23       ` Frederic Weisbecker
2015-02-06 18:51         ` Rik van Riel
2015-02-06 23:15           ` Frederic Weisbecker
2015-02-07  3:53             ` Rik van Riel
2015-02-07  6:34               ` Paul E. McKenney
2015-02-07  7:14                 ` Paul E. McKenney
2015-02-07  8:30                   ` Frederic Weisbecker
2015-02-07 11:29                     ` Rik van Riel
2015-02-07 20:06                     ` Paul E. McKenney
2015-02-09 15:42                       ` Rik van Riel
2015-02-05 20:23 ` [PATCH 2/5] rcu,nohz: run vtime_user_enter/exit only when state == IN_USER riel
2015-02-05 20:23 ` [PATCH 3/5] nohz,kvm: export context_tracking_user_enter/exit riel
2015-02-05 23:55   ` Paul E. McKenney
2015-02-05 20:23 ` [PATCH 4/5] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest riel
2015-02-05 23:56   ` Paul E. McKenney
2015-02-06 18:01   ` Frederic Weisbecker
2015-02-06 23:24   ` Frederic Weisbecker
2015-02-05 20:23 ` [PATCH 5/5] nohz: add stub context_tracking_is_enabled riel
2015-02-05 23:56   ` Paul E. McKenney
2015-02-06 13:46 ` [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest Frederic Weisbecker
2015-02-06 13:50   ` Paolo Bonzini
2015-02-06 16:19     ` Paul E. McKenney
2015-02-06 18:09     ` Frederic Weisbecker
2015-02-06 14:56   ` Rik van Riel [this message]
2015-02-06 18:05     ` Frederic Weisbecker
2015-02-06 15:00 ` Christian Borntraeger
2015-02-06 18:20   ` Frederic Weisbecker

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