linux-kernel - Re: [PATCH 0/7] hv: CPU onlining/offlining fixes and improvements

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Date:   Sat, 26 Nov 2016 09:05:57 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>
Subject: Re: [PATCH 0/7] hv: CPU onlining/offlining fixes and improvements

On Fri, 25 Nov 2016 13:48:36 +0100
Vitaly Kuznetsov <vkuznets@...hat.com> wrote:

> Some time ago we forbade CPU offlining for Hyper-V and this was sufficient
> if you boot with all CPUs onlined. Turns out, people may want to limit the
> number online CPUs by passing 'maxcpus=' kernel parameter and we hit a
> crash in Hyper-V code in this case. After some thinking, I think we may not
> only fix the crash but also make the offlining prevention fine-grained: we
> need to prevent from offlining CPUs which have VMBus channels attached
> only. All offlined CPUs may always be onlined.
> 
> PATCH1 fixes a bug which is not directly related to the series, I hit it
> while testing hv_vmbus module unload with this series.
> 
> Vitaly Kuznetsov (7):
>   hv: acquire vmbus_connection.channel_mutex in vmbus_free_channels()
>   hv: allocate synic pages for all present CPUs
>   hv: init percpu_list in hv_synic_alloc()
>   hv: change clockevents unbind tactics
>   hv: check all present cpus in vmbus_wait_for_unload()
>   hv: switch to cpuhp state machine for synic init/cleanup
>   hv: make CPU offlining prevention fine-grained
> 
>  drivers/hv/channel_mgmt.c |  6 +++--
>  drivers/hv/hv.c           | 60 ++++++++++++++++++++++++++++++++++++-----------
>  drivers/hv/hyperv_vmbus.h |  4 ++--
>  drivers/hv/vmbus_drv.c    | 28 ++++++++++++----------
>  4 files changed, 67 insertions(+), 31 deletions(-)
> 

As a temporary solution this is ok, but long term we need to support
dynamic CPU online/offline.