linux-kernel - Re: [Xen-devel] [PATCH 1/4] xen: disable non-boot VCPUs during suspend

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Message-ID: <51C2EBA7.9060208@citrix.com>
Date:	Thu, 20 Jun 2013 12:46:47 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	<xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/4] xen: disable non-boot VCPUs during suspend

On 20/06/13 11:38, Jan Beulich wrote:
>>>> On 19.06.13 at 17:25, David Vrabel <david.vrabel@...rix.com> wrote:
>> syscore_suspend() and syscore_resume() expect there to be only one
>> online CPU.  e.g., hrtimers_resume() only triggers events for the
>> current CPU.  Xen's suspend path was leaving all VCPUs online and then
>> attempting to fixup problems afterwards (e.g., with an explicit call
>> to clock_was_set() to trigger pending high resolution timers).
>>
>> Instead, disable non-boot CPUs before calling stop_machine() and
>> reenable them afterwards.
> 
> In XenoLinux the so called "fast suspend" mode was specifically
> added for performance reasons, and it looks like to date pv-ops
> only ever supported that mode. So one question is whether
> there's going to be any bad performance effect from this.

Yes :(

On a VM with 4 VCPUs, disable_boot_cpus() took > 200 ms.

I'll have to rethink this.

David
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