Message-ID: <54D4D62B.1040000@redhat.com>
Date: Fri, 06 Feb 2015 09:56:43 -0500
From: Rik van Riel <riel@redhat.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0
MIME-Version: 1.0
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
References: <1423167832-17609-1-git-send-email-riel@redhat.com> <20150206134631.GA21905@lerouge>
In-Reply-To: <20150206134631.GA21905@lerouge>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2089
Lines: 51

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iQEcBAEBAgAGBQJU1NYrAAoJEM553pKExN6DMgcIAIKoBBxsFEt7yOP+k32uqm+W
S/VbP5dIHE5OnYeBoitgNkia1U4rsAX6AVAuVFvKc7Y8aixENGzubPWHe0NuHida
VIaQmK92Jym4FH8Xsnj09MhgLV+ZEG/cCzdUFZfShJq3hHwzedZx+cC7uQMB6kd4
iuo7CtgTjzTgBce29Fc147azXlJbfFfwFt3a6YVxbv25IYpDL9ETulh34h6NrNLz
nB0snDjq8FHKcyjlD3XnJpT/tbaZcrZctExq4JrespcBMe6prMnoWvVoXWX/fVVG
TIR1hp2xfKWoS4gc56PnLazIIB9SRmlC/SzSMwAaSgf1dWa5BcwpuMbYVFIEeME=
=+BrK
-----END PGP SIGNATURE-----
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/