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Date: Fri, 04 Apr 2008 14:55:14 +0530
From: Balbir Singh <balbir@...ux.vnet.ibm.com>
To: Paul Menage <menage@...gle.com>
CC: Pavel Emelianov <xemul@...nvz.org>,
Hugh Dickins <hugh@...itas.com>,
Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
linux-kernel@...r.kernel.org, taka@...inux.co.jp,
linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [-mm] Add an owner to the mm_struct (v8)
Paul Menage wrote:
> On Fri, Apr 4, 2008 at 1:28 AM, Balbir Singh <balbir@...ux.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
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