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SubjectRe: [Lse-tech] Re: A common layer for Accounting packages

Marcelo Tosatti wrote:
> Yep, the netlink people should be able to help - they known what would be
> required for not sending messages in case there is no listener registered.
> Maybe its already possible? I have never used netlink myself.

If we notify the fork/exec/exit-events to user-space directly as you said,
I don't think some hackings on netlink is necessary.
For example, such packets is sent only when /proc/sys/.../process_grouping is set,
and user-side daemon set this value, and unset when daemon will exit.
It's not necessary to take too seriously.

>>And, why can't netlink packets send always?
>>If there are fork/exec/exit hooks, and they call CSA or other
>>process-grouping modules,
>>then those modules will decide whether packets for interaction with the
>>daemon should be
>>sent or not.
> The netlink data will be sent to userspace at fork/exec/exit hooks - one wants
> to avoid that if there are no listeners, so setups which dont want to run the
> accounting daemon dont pay the cost of building and sending the information
> through netlink.
> Thats what Andrew asked for if I understand correctly.

Does it mean "netlink packets shouled be sent to userspace unconditionally." ?
I have advocated steadfastly that fork/exec/exit hooks is necessary to support
process-grouping and to account per process-grouping.
It intend to be decided whether packets should be sent or not by hooked functions,
in my understanding.
Is it also one of the implementations whether using netlink-socket or not ?

>>In most considerable case, CSA's kernel-loadable-module using such hooks
>>will not be loaded
>>when no accounting daemon is running. Adversely, this module must be loaded
>>when accounting
>>daemon needs CSA's netlink packets.
>>Thus, it is only necessary to refer flag valiable and to execute
>>when no-accounting daemon is running.
> That would be one hack, although it is uglier than the pure netlink
> selection.

No, I can't agree this opinion.
It means netlink-packets will be sent unconditionally when fork/exec/exit occur.
Nobady can decide which packet is sent user-space, I think.

In addition, the definition of process grouping is lightweight in many cases.
For example, CpuSet can define own process-group by one increment-operation.

I think it's not impossible to implement it in userspace, but it's not reasonable.
An implementation as a kernel loadable module is reasonable and enough tiny.

>>In my estimation, we must pay additional cost for an increment-operation,
>>an decrement-op,
>>an comparison-op and an conditional jump-op. It's enough lightweight, I
>>For example:
>>If CSA's module isn't loaded, 'privates_for_grouping' will be empty.
>>inline int on_fork_hook(task_struct *parent, task_struct *newtask){
>> rcu_read_lock();
>> if( !list_empty(&parent->privates_for_grouping) ){
>> ..<Calling to any process grouping module>..;
>> }
>> rcu_read_unlock();
> Andrew has been talking about sending data over netlink to implement the
> accounting at userspace, so this piece of code is out of the game, no?

Indeed, I'm not opposed to implement the accounting in userspace and
using netlink-socket for kernel-daemon communication. But definition
of process-grouping based on any grouping policy should be done
in kernel space at reasonability viewpoint.

Linux Promotion Center, NEC
KaiGai Kohei <>
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