Re: [-mm] Add an owner to the mm_struct (v8)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Paul Menage wrote:
> On Fri, Apr 4, 2008 at 1:28 AM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>>  It won't uncharge for the memory controller from the root cgroup since each page
>>   has the mem_cgroup information associated with it.
> 
> Right, I realise that the memory controller is OK because of the ref counts.
> 
>>  For other controllers,
>>  they'll need to monitor exit() callbacks to know when the leader is dead :( (sigh).
> 
> That sounds like a nightmare ...
> 

Yes, it would be, but worth the trouble. Is it really critical to move a dead
cgroup leader to init_css_set in cgroup_exit()?

>>  Not having the group leader optimization can introduce big overheads (consider
>>  thousands of tasks, with the group leader being the first one to exit).
> 
> Can you test the overhead?
> 

I probably can write a program and see what the overhead looks like

> As long as we find someone to pass the mm to quickly, it shouldn't be
> too bad - I think we're already optimized for that case. Generally the
> group leader's first child will be the new owner, and any subsequent
> times the owner exits, they're unlikely to have any children so
> they'll go straight to the sibling check and pass the mm to the
> parent's first child.
> 
> Unless they all exit in strict sibling order and hence pass the mm
> along the chain one by one, we should be fine. And if that exit
> ordering does turn out to be common, then simply walking the child and
> sibling lists in reverse order to find a victim will minimize the
> amount of passing.
> 


Finding the next mm might not be all that bad, but doing it each time a task
exits, can be an overhead, specially for large multi threaded programs. This can
get severe if the new mm->owner belongs to a different cgroup, in which case we
need to use callbacks as well.

If half the threads belonged to a different cgroup and the new mm->owner kept
switching between cgroups, the overhead would be really high, with the callbacks
and the mm->owner changing frequently.

> One other thing occurred to me - what lock protects the child and
> sibling links? I don't see any documentation anywhere, but from the
> code it looks as though it's tasklist_lock rather than RCU - so maybe
> we should be holding that with a read_lock(), at least for the first
> two parts of the search? (The full thread search is RCU-safe).
> 

You are right about the read_lock()

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL