Re: [RFC] How to handle the rules engine for cgroups

From: KAMEZAWA Hiroyuki
Date: Fri Jul 18 2008 - 05:51:12 EST

On Tue, 1 Jul 2008 15:11:26 -0400
Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:

> Hi,
> While development is going on for cgroup and various controllers, we also
> need a facility so that an admin/user can specify the group creation and
> also specify the rules based on which tasks should be placed in respective
> groups. Group creation part will be handled by libcg which is already
> under development. We still need to tackle the issue of how to specify
> the rules and how these rules are enforced (rules engine).

A different topic.

Recently I'm interested in "How to write userland daemon program
to control group subsystem." To implement that effectively, we need
some notifier between user <-> kernel.

Can we use "inotify" to catch changes in cgroup (by daemon program) ?

For example, create a new file under memory cgroup
And a user watches the file by inotify.
The kernel modify modified-time of notify_at_memory_reach_limit file and call
fs/notify_user.c::notify_change() against this inode. He can catchthe event
by inotify.
(I think he can also catch removal of this file, etc...)

Is there some difficulty or problem ? (I'm sorry if we can do this now.)


> I have gathered few views, with regards to how rule engine can possibly be
> implemented, I am listing these down.
> Proposal 1
> ==========
> Let user space daemon hanle all that. Daemon will open a netlink socket
> and receive the notifications for various kernel events. Daemon will
> also parse appropriate admin specified rules config file and place the
> processes in right cgroup based on rules as and when events happen.
> I have written a prototype user space program which does that. Program
> can be found here. Currently it is in very crude shape.
> Various people have raised two main issues with this approach.
> - netlink is not a reliable protocol.
> - Messages can be dropped and one can loose message. That means a
> newly forked process might never go into right group as meant.
> - How to handle delays in rule exectuion?
> - For example, if an "exec" happens and by the time process is moved to
> right group, it might have forked off few more processes or might
> have done quite some amount of memory allocation which will be
> charged to the wring group. Or, newly exec process might get
> killed in existing cgroup because of lack of memory (despite the
> fact that destination cgroup has sufficient memory).
> Proposal 2
> ==========
> Implement one or more kernel modules which will implement the rule engine.
> User space program can parse the config files and pass it to module.
> Kernel will be patched only on select points to look for the rules (as
> provided by modules). Very minimal code running inside the kernel if there
> are no rules loaded.
> Concerns:
> - Rules can become complex and we don't want to handle that complexity in
> kernel.
> Pros:
> - Reliable and precise movement of tasks in right cgroup based on rules.
> Proposal 3
> ==========
> How about if additional parameters can be passed to system calls and one
> can pass destination cgroup as additional parameter. Probably something
> like sys_indirect proposal. Maybe glibc can act as a wrapper to pass
> additional parameter so that applications don't need any modifications.
> Concerns:
> ========
> - Looks like sys_indirect interface for passing extra flags was rejected.
> - Requires extra work in glibc which can also involve parsing of rule
> files. :-(
> Proposal 4
> ==========
> Some vauge thoughts are there regarding how about kind of freezing the
> process or thread upon fork, exec and unfreeze it once the thread has been
> placed in right cgroup.
> Concerns:
> ========
> - Requires reliable netlink protocol otherwise there is a possibility that
> a task never gets unfrozen.
> - On what basis does one freeze a thread. There might not be any rules to
> process for that thread we will unnecessarily delay it.
> Please provide your inputs regarding what's the best way to handle the
> rules engine.
> To me, letting the rules live in separate module/modules seems to be a
> reasonable way to move forward which will provide reliable and timely
> execution of rules and by making it modular, we can remove most of the
> complexity from core kernel code.
> Thanks
> Vivek
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