[systemd-devel] [PATCH] Add the CPU hotplug rule

[David Herrmann]

Hi

On Mon, Sep 15, 2014 at 3:19 PM, John Haxby <john.haxby at oracle.com> wrote:
> On 12/09/14 16:03, Kay Sievers wrote:
>> On Fri, Sep 12, 2014 at 3:04 PM, John Haxby <john.haxby at oracle.com> wrote:
>>> When I think of changing the
>>> behaviour of any removable hardware, udev is automatically where I look
>>> first. So if I'm missing something here, I would really like some more
>>> input.
>>
>> Udev does not decide which device show up, it just classifies stuff
>> that processes events.
>
> That's what I don't understand.
>
> Let's say I plug in a USB printer.  Looking through the udev rules on my
> machine I see
>
>     SUBSYSTEM=="printer", TAG+="systemd"
>        ENV{SYSTEMD_WANTS}+="printer.target"
>
> (line split for readability).    A whileback I proposed
>
>    ACTION==“add” KERNEL==“cpu[0-9]*” TAG+="systemd"
>        ENV{SYSTEMD_WANTS}+=“cpuadd.service”
>
> Aside from the obvious syntactic differences, what's the difference
> between these?

One is a target, the other is a service. There is a fundamental
difference between both. A target does not cause an immediate action,
a service does. The target just hooks up the udev event into systemd,
it does not associate any default action. The cpuadd.service, on the
other hand, does add a default action.

> Does it make any difference if the default is not to bring online more
> CPUs that the system booted with?  Or some other clever default?
>
> Why does the kernel not bring CPUs online automatically?  This must have
> come up before, and what you have been saying seems to suggest that
> there is something going on here that I am just missing.

The idea is to add a kernel runtime option (like sysctl) which is
named "cpu_auto_on" or something like that. Or make it a sysfs file in
/sys/bus/cpu/ just like "drivers_autoprobe". This should be set to "1"
by default, thus, all CPUs get activated without any udev interaction
(if you want to avoid races during bootup, use the kernel
command-line.. or a static kernel config option..).
Everybody else is free to set this option to "0" and write their own
udev rules. This way, udev does not have to bother with default
actions.

Is there anything wrong with this proposed solution? Or why do you
insist on your solution?

Thanks
David