Message-Id: <200804222230.25743.tomasz@grobelny.oswiecenia.net>
Date:	Tue, 22 Apr 2008 22:30:25 +0200
From:	Tomasz Grobelny <tomasz@...belny.oswiecenia.net>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	dccp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [DCCP] [RFC] [Patchv2 1/1]: Queuing policies -- reworked version of Tomasz's patch set

Dnia Tuesday 22 of April 2008, Gerrit Renker napisał:
> | > | While we are developing in the test tree compatibility is not
> | > | important at all. But real world needs compatibility, especially if
> | > | it's not that expensive. Otherwise people will get weird behaviour
> | > | without any messages indicating what is wrong. And it's certainly not
> | > | how code should be written. It can't be that binary package of VLC
> | > | (or any other streaming server) cannot use newer kernel version
> | > | because we added a new field.
> | >
> | > True, but we need to get something working first. There is no point
> | > exporting an API which only works half.
> |
> | You can never assume that the API is feature complete. Even if it "works
> | half" at the beginning (which is I guess quite normal) it should be easy
> | to make it "work full" without breaking compatibility when it's not
> | necessary.
>
> No I don't agree. Things that work half are worse than constant failures,
> since it is never clear when they will fail next. But maybe you didn't mean
> that.
>
I mean that what we are currently discussing (only priorities) is in itself 
useful ("works half"). Adding more features (expiry times) would be a nice 
but non essential feature ("work full").

(a bit of reordering)
> So we can reduce this to the other question "is 32 bit enough for priority
> information".
>
That is an important question. But I'm afraid the answer is no (even though I 
thought otherwise when writing previous mail). When we want to add packet 
expiry times we will need a field for timestamp. Which is quite big (64 
bits?). But note that it is only needed for in-kernel implemetation. 
Userspace can write this information on as low as 16 bits. This leads me to 
conclusions stated below.

> | > Semantically there is no restriction of what the u32 number represents.
> | > And it is a large space. I don't think that a larger size is required.
> |
> | Ok, for now 32 bits are enough and I can't think of the need to use more
> | bits. I wouldn't be so sure about future though. But ok, if it turns up
> | that we need more space in the future then we will see, there is no need
> | to worry about it now. We can indeed assume 32 bits for now.
>
> So it only remains to see whether to use a field in dccp_skb_cb, or to
> use skb->priority. I think the latter has advantages, since this field
> is only set/used for the SO_PRIORITY field (in socket(7)).
>
I think we should distingish two things:
1. How do we pass data along with each packet from userspace to kernelspace.
2. How do we store data in in-kernel skb for internal use.
I'm afraid there was no such separation in my patches and mixing those two 
things causes a lots misunderstandings.

Ad 1. In this case we need not care about sizes etc. Clean and extensible 
interface is very important. We can also have very different priority data 
representantion from what is later stored in skb.
Ad 2. In this case sizes do matter. We can use whichever fields we want. In 
particular we need not keep all the data passed from userspace application in 
one chunk but we can distribute it among arbitrary skb fields.

So this could look like that:
Userspace application fills in a structure:
struct dccp_packet_info {
	s8 priority; //as it is now
	u16 expire_after; //up to 65s with 1ms accuracy
}
and passes it to sendmsg(..., struct dccp_packet_info pktinfo), which passes 
it to qpolicy_push which passes it to qpolicy_prio_push which does the tricky 
stuff with CMSG and does:
skb->priority=pktinfo->priority;
skb->tstamp=now()+pktinfo->expire_after;
(possibly other things that could potentially store data in dccp_skb_cb)

This would allow us to:
1. Use existing skb fields in a clean way. I mean according to their names 
(priority, tstamp).
2. Have a clean userspace interface not affected by internal kernel 
implementation details.

What do you think of such decoupling?
-- 
Regards,
Tomasz Grobelny
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