linux-kernel - Re: [PATCH V2] Driver cpu: update online when cpu

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Message-ID: <CALZtONCwrBb+HPY7j7x4R-xmyhajFNCwaS3sns50RMDxHxyiEQ@mail.gmail.com>
Date:	Wed, 29 Oct 2014 17:52:10 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Neil Zhang <zhangwm@...vell.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	nfont@...ux.vnet.ibm.com
Subject: Re: [PATCH V2] Driver cpu: update online when cpu_up/down besides sysfs

On Wed, Oct 29, 2014 at 5:46 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Monday, October 27, 2014 08:46:08 PM Dan Streetman wrote:
>> On Mon, Oct 27, 2014 at 5:38 PM, Rafael J. Wysocki
>> <rafael.j.wysocki@...el.com> wrote:
>> > On 10/27/2014 3:59 AM, Neil Zhang wrote:
>> >>
>> >> The current per-cpu offline info won't be updated when we use
>> >> any other method besides sysfs to call cpu_up/down.
>> >> Thus the cpu/online can't reflect the real online status.
>> >>
>> >> This patch is going to fix the issue introduced by commit
>> >> 0902a9044fa5b7a0456ea4daacec2c2b3189ba8c (Driver core:
>> >> Use generic offline/online for CPU offline/online)
>> >>
>> >> CC: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> >> Tested-by: Dan Streetman <ddstreet@...e.org>
>> >> Signed-off-by: Neil Zhang <zhangwm@...vell.com>
>> >
>> >
>> > Oh dear, no.
>> >
>> > Please first tell me what exactly the problem you're seeing is.
>>
>> For some background, here is my last comment on the first email thread on this:
>> https://lkml.org/lkml/2014/10/27/595
>>
>> I didn't create this patch, but the problem essentially is that before
>> your commit the individual cpu online nodes
>> (/sys/devices/system/cpu/cpuN/online) stayed in sync during
>> cpu_down/up, because they used the cpu_online_mask; while after the
>> commit, they are tracked by the cpu's generic dev->offline flag, which
>> isn't updated during cpu_down/up.
>
> Which is not triggered from sysfs.
>
>> So now, any place in the kernel
>> that brings a cpu up or down must also update the cpu->dev->offline
>> flag.
>
> Not any place.  In particular, system suspend-resume doesn't need to
> do that, because it takes CPUs offline and then brings them back
> online.
>
> If there's a place in the kernel where CPUs are taken offline and
> left in that state, then it needs to be updated.

The only place I know of is ppc's dlpar code, as mentioned below.

Neil, as you crafted the original patch, I assume you know of some
other place in the kernel doing cpu_up/down directly, where you're
seeing this problem?

>
>> My interest in the patch was coincidental because I was seeing
>> the same problem when using dlpar operations to hotplug cpus, which
>> uses the arch/powerpc/platform/pseries/dlpar.c code; that code brings
>> a cpu offline when it's hot-removed (and the cpu online when it's
>> hot-added), but it hasn't been changed to also update the cpu's
>> dev->offline flag.
>
> It should be modified to do that.
>
>> As I said in the previous email to the first thread, the ppc dlpar
>> operation might be changed in the future to fully unregister a cpu
>> when it's hot-removed, which would remove the entire sysfs cpuN
>> directory.  Alternately and/or until then, it could be updated to
>> simply update the cpu'd dev->offline flag (that's what I originally
>> did for my own testing).  However, without a central place to update
>> the cpu's dev->offline field, like this, or possibly in
>> set_cpu_online(), or elsewhere during cpu_down/up, each place in the
>> kernel that calls cpu_down() or cpu_up() also needs to update the
>> dev->offline flag.  It's possible that the ppc dlpar code is the only
>> place in the kernel that has this problem; I haven't searched.
>
> It is quite likely to be the only place like that.
>
> While I'm not familiar with the code in question, the most straightforward
> way to fix the problem would be to replace cpu_down() in there with
> device_offline(get_cpu_device(cpu)), but that needs to be called under
> device_hotplug_lock.

Ok, will do.

>
> --
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
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