[Devel] Re: [PATCH 1/2] rcfs core patch

[Herbert Poetzl]

On Thu, Mar 08, 2007 at 03:43:47PM +0530, Srivatsa Vaddagiri wrote:
> On Wed, Mar 07, 2007 at 08:12:00PM -0700, Eric W. Biederman wrote:
> > The review is still largely happening at the why level but no
> > one is addressing that yet.  So please can we have a why.
> 
> Here's a brief summary of what's happening and why. If its not clear,
> pls get back to us with specific questions.
> 
> There have been various projects attempting to provide resource
> management support in Linux, including CKRM/Resource Groups and UBC.

let me note here, once again, that you forgot Linux-VServer
which does quite non-intrusive resource management ...

> Each had its own task-grouping mechanism. 

the basic 'context' (pid space) is the grouping mechanism
we use for resource management too

> Paul Menage observed [1] that cpusets in the kernel already has a
> grouping mechanism which was working well for cpusets. He went ahead
> and generalized the grouping code in cpusets so that it could be used
> for overall resource management purpose. 

> With his patches, it is possible to even create multiple hierarchies
> of groups (see [2] on why multiple hierarchies) as follows:

do we need or even want that? IMHO the hierarchical
concept CKRM was designed with, was also the reason
for it being slow, unuseable and complicated

> mount -t container -o cpuset none /dev/cpuset	<- cpuset hierarchy
> mount -t container -o mem,cpu none /dev/mem	<- memory/cpu hierarchy
> mount -t container -o disk none /dev/disk	<- disk hierarchy
> 
> In each hierarchy, you can create task groups and manipulate the
> resource parameters of each group. You can also move tasks between
> groups at run-time (see [3] on why this is required). 

> Each hierarchy is also manipulated independent of the other.          

> Paul's patches also introduced a 'struct container' in the kernel,
> which serves these key purposes:
> 
> - Task-grouping
>   'struct container' represents a task-group created in each hierarchy.
>   So every directory created under /dev/cpuset or /dev/mem above will
>   have a corresponding 'struct container' inside the kernel. All tasks
>   pointing to the same 'struct container' are considered to be part of
>   a group
> 
>   The 'struct container' in turn has pointers to resource objects which
>   store actual resource parameters for that group. In above example,
>   'struct container' created under /dev/cpuset will have a pointer to
>   'struct cpuset' while 'struct container' created under /dev/disk will
>   have pointer to 'struct disk_quota_or_whatever'.
> 
> - Maintain hierarchical information
>   The 'struct container' also keeps track of hierarchical relationship
>   between groups.
> 
> The filesystem interface in the patches essentially serves these
> purposes:
> 
> 	- Provide an interface to manipulate task-groups. This includes
> 	  creating/deleting groups, listing tasks present in a group and 
> 	  moving tasks across groups
> 
> 	- Provdes an interface to manipulate the resource objects
> 	  (limits etc) pointed to by 'struct container'.
> 
> As you know, the introduction of 'struct container' was objected
> to and was felt redundant as a means to group tasks. Thats where I
> took a shot at converting over Paul Menage's patch to avoid 'struct
> container' abstraction and insead work with 'struct nsproxy'.

which IMHO isn't a step in the right direction, as
you will need to handle different nsproxies within
the same 'resource container' (see previous email)

> In the rcfs patch, each directory (in /dev/cpuset or /dev/disk) is
> associated with a 'struct nsproxy' instead. The most important need
> of the filesystem interface is not to manipulate the nsproxy objects
> directly, but to manipulate the resource objects (nsproxy->ctlr_data[]
> in the patches) which store information like limit etc.
> 
> > I have a question?  What does rcfs look like if we start with
> > the code that is in the kernel?  That is start with namespaces
> > and nsproxy and just build a filesystem to display/manipulate them?
> > With the code built so it will support adding resource controllers
> > when they are ready?
> 
> If I am not mistaken, Serge did attempt something in that direction,
> only that it was based on Paul's container patches. rcfs can no doubt
> support the same feature.
> 
> > >  	struct ipc_namespace *ipc_ns;
> > >  	struct mnt_namespace *mnt_ns;
> > >  	struct pid_namespace *pid_ns;
> > > +#ifdef CONFIG_RCFS
> > > +	struct list_head list;
> > 
> > This extra list of nsproxy's is unneeded and a performance problem the
> > way it is used.  In general we want to talk about the individual resource
> > controllers not the nsproxy.
> 
> I think if you consider the multiple hierarchy picture, the need
> becomes obvious.
> 
> Lets say that you had these hierarchies : /dev/cpuset, /dev/mem, /dev/disk
> and the various resource classes (task-groups) under them as below:
> 
> 	/dev/cpuset/C1, /dev/cpuset/C1/C11, /dev/cpuset/C2
> 	/dev/mem/M1, /dev/mem/M2, /dev/mem/M3
> 	/dev/disk/D1, /dev/disk/D2, /dev/disk/D3
> 
> The nsproxy structure basically has pointers to a resource objects in
> each of these hierarchies. 
> 
> 	nsproxy { ..., C1, M1, D1} could be one nsproxy
> 	nsproxy { ..., C1, M2, D3} could be another nsproxy and so on
> 
> So you see, because of multi-hierachies, we can have different
> combinations of resource classes.
> 
> When we support task movement across resource classes, we need to find a
> nsproxy which has the right combination of resource classes that the
> task's nsproxy can be hooked to.

no, not necessarily, we can simply create a new one
and give it the proper resource or whatever-spaces

> That's where we need the nsproxy list. Hope this makes it clear.
> 
> > > +	void *ctlr_data[CONFIG_MAX_RC_SUBSYS];
> > 
> > I still don't understand why these pointers are so abstract,
> > and why we need an array lookup into them?
> 
> we can avoid these abstract pointers and instead have a set of pointers
> like this:
> 
> 	struct nsproxy {
> 		...
> 		struct cpu_limit *cpu;	/* cpu control namespace */
> 		struct rss_limit *rss;	/* rss control namespace */
> 		struct cpuset *cs;	/* cpuset namespace */
> 
> 	}
> 
> But that will make some code (like searching for a right nsproxy when a
> task moves across classes/groups) very awkward.
> 
> > I'm still inclined to think this should be part of /proc, instead of a purely
> > separate fs.  But I might be missing something.
> 
> A separate filesystem would give us more flexibility like the
> implementing multi-hierarchy support described above.

why is the filesystem approach so favored for this
kind of manipulations?

IMHO it is one of the worst interfaces I can imagine
(to move tasks between spaces and/or assign resources)
but yes, I'm aware that filesystems are 'in' nowadays

best,
Herbert

> -- 
> Regards,
> vatsa
> 
> 
> References:
> 
> 1. http://lkml.org/lkml/2006/09/20/200 
> 2. http://lkml.org/lkml/2006/11/6/95
> 3. http://lkml.org/lkml/2006/09/5/178
> 
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