[systemd-devel] systemd and kernel process

[Kay Sievers]

On Thu, Oct 1, 2015 at 9:30 PM, Steve Dickson <SteveD at redhat.com> wrote:
>
>
> On 10/01/2015 09:24 AM, Kay Sievers wrote:
>> On Wed, Sep 30, 2015 at 10:49 PM, Steve Dickson <SteveD at redhat.com> wrote:
>>> Is there a way for systemd to monitor kernel process?
>>> By monitor I mean the existence.
>>
>> No, and there is no plan to do anything like that.
>>
>> Kernel tasks are kernel internals, and userspace must not make any
>> assumptions about them. They can come and go at any time between any
>> kernel versions.
>>
>> Custom tools can do what they need, but systemd should not offer to do
>> that to users.
> First all thanks for the response
>
> kernel process in question is nfsd. The number of thread
> is kept in /proc/fs/nfsd/threads.
>
> So the idea would be doing a systemctl status nfsd
> and number in /proc/fs/nfsd/threads is zero the
> service would be deactive. An non-zero number the
> service would active.
>
> Is this something systemd could be used for?

No, not directly. There is no facility to watch /proc or any other
similar interface for such changes. Plain /proc directories are just
not capable of providing event information to userspace.

The kernel's nfs implementation would need a character device where
events are send out, or possibly a poll()able file in /proc, or
something in /sys/devices/, or a similar approach, where udev can
react to. Such interface could be used to signal systemd that
userspace should react to state changes in the kernel.

Kay