Message-Id: <200805262340.53355.tomasz@grobelny.oswiecenia.net>
Date:	Mon, 26 May 2008 23:40:53 +0200
From:	Tomasz Grobelny <tomasz@...belny.oswiecenia.net>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>, acme@...hat.com
Cc:	dccp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] [DCCP][QPOLICY]: Make information about qpolicies available to userspace

Dnia Monday 26 of May 2008, Gerrit Renker napisał:
> | I have nothing against creating generic interface for obtaining
> | information about various aspects of dccp. But I can't see how netlink
> | can help us here. From the documents I've read I understand that generic
> | netlink requires you to create a special kind of socket type and then you
> | can send or receive information through it. But it seems to be something
> | separate from dccp sockets.
>
> Yes, the netlink socket would be different from the DCCP socket and it
> is actually a good point - it shows that this interface can only be used
> for global parameters or settings. Or maybe there is a way of
> associating both sockets.
>
I haven't seen any. But I'm quite new to kernel development (and to netlink in 
particular).

> | As for information we can get from kernel wrt dccp we have at least:
> | 1. fixed information that depends only on kernel version. For example
> | list of ccids, list of available qpolicies, list of parameters for given
> | qpolicy, etc. These are system wide and don't need reference to socket.
> | These could even be exposed by read-only entries in /sys or /proc.
>
> That is a good point - I think Arnaldo had a similar idea once for
> implenting system-wide policies regarding which CCIDs are supported.
> Something like net.ipv4.tcp_available_congestion_control.
>
Ok, we may go this way. I'll try to write a patch in a few days (quite busy 
now).

> | 3. statistics. These are not options in the sense that you cannot set
> | them. You also cannot expect that two consecutive reads will return the
> | same value. They depend on certain asynchronous events in various parts
> | of dccp code. The first question is: should we pass those raw events to
> | applications (such as "prio qpolicy: packet dropped") or aggregate data
> | ("in the last 100 packets 17 were dropped before sending")? For this kind
> | of information we need reference to socket (struct sock *). And I can't
> | see how we could obtain it from functions not mentioned in struct proto.
>
> It was for this part that I looked at netlink. This goes into the
> direction of a new API or at least API extensions.
> 
We basically need a callback from code that can access struct sock...

> (...)
> For per-socket statistics there is indeed a need for a notification
> mechanism. If a synchronous mechanism is sufficient, then something like
> the current CCID-3 getsockopt(DCCP_SOCKOPT_CCID_{RX,TX}_INFO) can be used.
>
Synchronous mechanism is acceptable but callback (at least as an option) would 
be way nicer.

> For asynchronous notification, using a special signal comes to mind,
> where the application installs a signal handler as notifier.
>
We'd have to pass affected socket identifier (process may use more than one 
socket). To me it looks like a not-so-nice hack. But still I don't have 
better ideas on how to implement asynchronous event notification (callback). 
Ok, but if we are at hackish ideas what about this one: use dccp_ioctl to 
wait on a mutex. If an event occurs we release mutex thus allowing dccp_ioctl 
to finish execution and consequently informing user that something 
interesting has just happened - what it exactly is would have to be 
determined by other means.

> The best that I can think of is to map a segment of userspace memory
> into the kernel and use this memory area as a (ready-only) statistics
> struct. That would save the system call overhead of using getsockopt().
>
I don't think the syscall overhead is big enough to care. After all you don't 
need statistics that often. Shared memory creates potential problems with 
synchronization (ok, it is read-only but you need to provide consistent 
data). Plus you must update them whether needed or not because you never know 
when the user will want to read them (it may be a problem in some scenarios 
and may not be a problem in others). IMO more problems that it's worth.
-- 
Regards,
Tomasz Grobelny
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