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


    On 06/02/2015 14:46, Frederic Weisbecker 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.

    Yeah, it's a new thing but Marcelo, Luiz and Rik have been having a lot
    of fun with them (with PREEMPT_RT too). They're getting pretty good
    results given the right tuning.

    I'll let Paul queue the patches for 3.21 then!

    Paolo

    > That said it's potentially
    > interesting to turn off the tick on the host when the guest runs.


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