Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest

[Rik van Riel]

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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@xxxxxxxxxx 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.


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