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    SubjectRe: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12
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    On 19/04/17 13:08, Peter Zijlstra wrote:
    > On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote:
    >> On 04/19/2017 01:28 PM, Peter Zijlstra wrote:
    >>>
    >>> So the thing Maz complained about is because KVM assumes
    >>> synchronize_srcu() is 'free' when there is no srcu_read_lock() activity.
    >>> This series 'breaks' that.
    >>
    >> Why is such a behaviour change not mentioned in the cover letter?
    >> I could not find anything in the patch descriptions that would
    >> indicate a slowdown. How much slower did it get?
    >>
    >> But indeed, there are several places at KVM startup which have been
    >> reworked to srcu since normal rcu was too slow for several usecases.
    >> (Mostly registering devices and related data structures at startup,
    >> basically the qemu/kvm coldplug interaction)
    >
    > I suspect Paul is not considering this a 'normal' RCU feature, and
    > therefore didn't think about changing this.
    >
    > I know I was fairly surprised by this requirement when I ran into it;
    > and only accidentally remembered it now that maz complained.

    The issue I noticed yesterday has been addressed here:

    https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=dev.2017.04.17a&id=6eec94fe40e294b04d32c8ef552e28fa6159bdad

    and was triggered by the constant mapping/unmapping of memslots that
    QEMU triggers when emulating a NOR flash that UEFI uses for storing its
    variables.

    So far, I'm not seeing any other spectacular regression introduced by
    this series.

    Thanks,

    M.
    --
    Jazz is not dead. It just smells funny...

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