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Date:	Sat, 19 Apr 2008 22:42:32 +0200
From:	Tomasz Grobelny <tomasz@...belny.oswiecenia.net>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>, 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 Friday 18 of April 2008, Gerrit Renker napisaƂ:
> | > There is a more serious problem here: apparently no one tried to
> | > compile the `qpolicy' subtree since the changes on Monday. It fails
> | > with the BUILD_BUG_ON.
> | >
> | > I tried yesterday evening and found that there is not even a
> | > possibility of adding a single field to struct dccp_skb_cb: with the
> | > addition of the inet{,6}_skb_parm, the struct has reached its maximum
> | > size of 48 bytes (try printk("%u", sizeof(struct dccp_skb_cb));).
> | >
> | > There is a solution to this, will post a revised patch shortly.
> |
> | What you proposed in the patch should work ok for qpolicies for now. But
> | sooner or later the need to add a field to struct dccp_skb_cb will arise.
> | So maybe we should think about it now...
> | Other possibility apart from what you proposed in the patch is creating
> | struct dccp_skb_cb_ext, move to it some fields from struct dccp_skb_cb
> | and in struct add a pointer to this new struct dccp_skb_cb_ext. Ok, maybe
> | it is not the prettiest idea but in case somebody needs yet another
> | additional field in struct dccp_skb_cb_ext it would be nice to have an
> | idea how to do it.
>
> When the patch failed to compile I thought about those alternatives.
> Trying to extend the dccp_skb_cb over and above what is in there will be
> messy, since the IPv4/v6 parameters are required by other subsystems.
>
If inet{,6}_skb_parm is used only outside DCCP code then why at all should it 
be placed in struct dccp_skb_cb taking up quite a lot of valuable space? Why 
not put it directly in struct sk_buff? Especially that it is present in 
struct udp_skb_cb, struct tcp_skb_cb as well.

> | > With regard to your point above: I think there is a way of keeping the
> | > framework open without bending over backwards to ensure the binary
> | > compatibility.
> |
> | 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.

> In this regard, another problem with the patch has arisen: the type of
> passing priority information will not work on 64-bit architectures in
> compatibility mode.
>
> Thanks to an answer by David Miller I found this out today. You can
> verify this problem by running the patch e.g. on a sparc64 system: no
> packets will be sent, sendmsg() returns with EINVAL.
>
I don't have any 64 bit system at hand...
Does David's suggestion mean that changes are only required in userspace 
application or on the kernel side as well?

> | > My question for you in this regard: which policy can you think of whose
> | > priorities/preferences can not be mapped into an unsigned 32-bit
> | > integer? I think that such a number is sufficiently expressive enough
> | > to cater for a wide range of policies. It can be interpreted as
> | >  * symbolic value (enum)
> | >  * ascii string (similar to DCCP service code)
> | >  * millisecond timeout (with room for up to 7 weeks)
> | >  * microsecond timeout (with room for up to 1.19 hours)
> | >  * priority values (the large range 0..2^32-1 can be partitioned)
> | >  * ... ?
> |
> | Putting any of those values in an integer is pretty straightforward. But
> | when you want to put both timeout and priority things get messy. Would it
> | be possible at least to use structure like that:
> | struct dccp_packet_info
> | {
> | 	s8	priority;
> | 	u16	timeout_mantissa:10;
> | 	u16	timeout_exponent:6;
> | }
> | ? That would still fit in 32-bit integer (so could be stored in
> | skb->priority), but would provide a much cleaner interface.
> | --
>
> Yes that is possible. It need not be a struct, the same effect can be
> achieved with bit shifts and bitmasks (this is the way the Ack Vectors
> are encoded).
>
Yes, it can be achieved by bit operations. But to me a structure is a much 
cleaner way. And that is especially important when designing interfaces. 
Final effects should be the same but when using structure it is the compiler 
that takes care of low level bit operations.

> 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.
-- 
Regards,
Tomasz Grobelny
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