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

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Message-ID: <MWHPR03MB26692E612C3599E12BF7C8DBBF8A0@MWHPR03MB2669.namprd03.prod.outlook.com>
Date:   Mon, 28 Nov 2016 06:56:23 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH 0/7] hv: CPU onlining/offlining fixes and improvements

> From: Stephen Hemminger [mailto:stephen@...workplumber.org]
> Sent: Sunday, November 27, 2016 01:06
> To: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: devel@...uxdriverproject.org; linux-kernel@...r.kernel.org; KY 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.
> >
> 
> As a temporary solution this is ok, but long term we need to support
> dynamic CPU online/offline.

If a CPU is bound to some channels, it seems impossible to make it offline,
unless Hyper-V supplies a mechanism to dynamically rebind  the channels (i.e.
without closing & opening the channels) to another CPU, e.g. CPU0.
Currently it looks there is no such mechanism.

For CPU online, my understanding is: this patchset of Vitaly actually makes it
work (see PATCH 6/7).

Thanks,
-- Dexuan