Re: [RFC][UPDATED PATCH 2.6.16] [Patch 9/9] Generic netlink interface for delay accounting

[jamal <hadi@cyberus.ca>]

On Sat, 2006-25-03 at 15:11 +0530, Balbir Singh wrote:

> 
> Thanks for the advice, I will dive into nesting. I could not find any 
> in tree users who use nesting, so I have a few questions
> 

Hrm - I have to say i am suprised theres nothing; i could have sworn
Thomas had done some conversions already.

> nla_nest_start() accepts two parameters an skb and an attribute type.
> Do I have to create a new attribute type like TASKSTATS_TYPE_AGGR to
> contain the nested attributes 
> 
> TASKSTATS_TYPE_AGGR
>    TASKSTATS_TYPE_PID/TGID
>    TASKSTATS_TYPE_STATS
> 
>
> but this will lead to
> 
> TASKSTATS_TYPE_AGGR
>    TASKSTATS_TYPE_PID
>    TASKSTATS_TYPE_STATS
> TASKSTATS_TYPE_AGGR
>    TASKSTATS_TYPE_TGID
>    TASKSTATS_TYPE_STATS
> 
> being returned from taskstats_exit_pid().
> 

no this is wrong by virtue of having TASKSTATS_TYPE_AGGR twice.
Again invoke the rule i cited earlier.
What you could do instead is a second AGGR; and your nesting would be:

TASKSTATS_TYPE_AGGR1 <--- nest start with this type
   TASKSTATS_TYPE_PID <-- NLA_U32_PUT
   TASKSTATS_TYPE_STATS <-- NAL_PUT_TYPE
                     <-- nest end of TASKSTATS_TYPE_AGGR1
TASKSTATS_TYPE_AGGR2 <--- nest start with this type
   TASKSTATS_TYPE_TGID <-- NLA_U32_PUT
   TASKSTATS_TYPE_STATS <-- NAL_PUT_TYPE
                       <-- nest end of TASKSTATS_TYPE_AGGR2

> The other option is to nest
> 
> TASKSTATS_TYPE_PID/TGID
>    TASKSTATS_TYPE_STATS
> 

The advantage being you dont introduce another T.

> but the problem with this approach is, nla_len contains the length of
> all attributes including the nested attribute. So it is hard to find
> the offset of TASKSTATS_TYPE_STATS in the buffer.
> 

So you would distinguish the two as have something like:

TASKSTATS_TYPE_PID
   u32 pid
   TASKSTATS_TYPE_STATS
TASKSTATS_TYPE_TGID
   u32 tgid
   TASKSTATS_TYPE_STATS
or
TASKSTATS_TYPE_PID
   u32 pid
TASKSTATS_TYPE_TGID
   u32 tgid

both should be fine. The difference between the two is the length in the
second case will be 4 and in the other case will be larger. 

But come to think of it, this will introduce unneeded semantics; you
have very few items to do, so forget it. Go with scheme #1 but change
the names to TASKSTATS_TYPE_AGGR_PID and TASKSTATS_TYPE_AGGR_TGID.

cheers,
jamal