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

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


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