Re: [Lse-tech] Re: [Patch 7/7] Generic netlink interface (delay accounting)

[jamal <hadi@cyberus.ca>]

On Mon, 2006-06-03 at 12:00 -0500, Shailabh Nagar wrote:

> My design was to have the listener get both responses (what I call 
> replies in the code) as well as events (data sent on exit of pid)
> 

I think i may not be doing justice explaining this, so let me be more
elaborate so we can be in sync.
Here is the classical way of doing things:

- Assume several apps in user space and a target in the kernel (this
could be reversed or combined in many ways, but the sake of
simplicity/clarity make the above assumption).
- suppose we have five user space apps A, B, C, D, E; these processes
would typically do one of the following class of activities:

a) configure (ADD/NEW/DEL etc). This is issued towards the kernel to
set/create/delete/flush some scalar attribute or vector. These sorts of
commands are synchronous. i.e you issue them, you expect a response
(which may indicate success/failure etc). The response is unicast; the
effect of what they affected may cause an event which may be multicast.

b) query(GET). This is issued towards the kernel to query state of
configured items. These class of commands are also synchronous. There
are special cases of the query which dump everything in the target -
literally called "dumps". The response is unicast.

c) events. These are _asynchronous_ messages issued by the kernel to
indicate some happening in the kernel. The event may be caused by #a
above or any other activity in the kernel. Events are multicast.
To receive them you have to register for the multicast group. You do so
via sockets. You can register to many multicast group.

For clarity again assume we have a multicast group where announcements
of pids exiting is seen and C and D are registered to such a multicast
group.
Suppose process A exits. That would fall under #c above. C and D will be
notified.
Suppose B configures something in the kernel that forces the kernel to
have process E exit and that such an operation is successful. B will get
acknowledgement it succeeded (unicast). C and D will get notified
(multicast). 
Suppose C issued a GET to find details about a specific pid, then only C
will get that info back (message is unicast).
[A response message to a GET is typically designed to be the same as an
ADD message i.e one should be able to take exactly the same message,
change one or two things and shove back into the kernel to configure].
Suppose D issued a GET with dump flag, then D will get the details of
all pids (message is unicast).

Is this clear? Is there more than the above you need?

There are no hard rules on what you need to be multicasting and as an
example you could send periodic(aka time based) samples from the kernel
on a multicast channel and that would be received by all. It did seem
odd that you want to have a semi-promiscous mode where a response to a
GET is multicast. If that is still what you want to achieve, then you
should.

> However, we could switch to the model you suggest and use a 
> multithreaded send/receive userspace utility.
> 

This is more of the classical way of doing things. 


> >There is a recent netlink addition to make sure
> >that events dont get sent if no listeners exist.
> >genetlink needs to be extended. For now assume such a thing exists.
> >  
> >
> Ok. Will this addition work for both unicast and multicast modes ?
> 

If you never open a connection to the kernel, nothing will be generated
towards user space. 
There are other techniques to rate limit event generation as well (one
such technique is a nagle-like algorithm used by xfrm).

> >
> Will this be necessary ? Isn't genl_rcv_msg() going to return a -EOPNOTSUPP
> automatically for us since we've not registered the command ?
>  

Yes, please in your doc feedback remind me of this,

> >
> >Also if you can provide feedback whether the doc i sent was any use
> >and what wasnt clear etc.
> >  
> >
> Will do.
> 

also take a look at the excellent documentation Thomas Graf has put in
the kernel for all the utilities for manipulating netlink messages and
tell me if that should also be put in this doc (It is listed as a TODO).


cheers,
jamal

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