Re: [PATCH RFC] fsio: filesystem io accounting cgroup

[Sha Zhengju <handai.szj@gmail.com>]

On Wed, Jul 10, 2013 at 2:03 PM, Konstantin Khlebnikov
<khlebnikov@openvz.org> wrote:
> Sha Zhengju wrote:
>>
>> Hi,
>>
>> On Mon, Jul 8, 2013 at 5:59 PM, Konstantin Khlebnikov
>> <khlebnikov@openvz.org>  wrote:
>>>
>>> >  This is proof of concept, just basic functionality for IO controller.
>>> >  This cgroup will control filesystem usage on vfs layer, it's main goal
>>> > is
>>> >  bandwidth control. It's supposed to be much more lightweight than
>>> > memcg/blkio.
>>> >
>>> >  This patch shows easy way for accounting pages in dirty/writeback
>>> > state in
>>> >  per-inode manner. This is easier that doing this in memcg in per-page
>>> > manner.
>>> >  Main idea is in keeping on each inode pointer (->i_fsio) to cgroup
>>> > which owns
>>> >  dirty data in that inode. It's settled by fsio_account_page_dirtied()
>>> > when
>>> >  first dirty tag appears in the inode. Relying to mapping tags gives us
>>> > locking
>>> >  for free, this patch doesn't add any new locks to hot paths.
>>
>> While referring to dirty/writeback numbers, what I care about is 'how
>> many dirties in how many memory' and later may use the proportion to
>> decide throttling or something else. So if you are talking about nr of
>> dirty pages without memcg's amount of memory, I don't see the meaning
>> of a single number.
>
>
> I'm planning to add some thresholds or limits to fsio cgroup -- how many
> dirty pages
> this cgroup may have. memcg is completely different thing: memcg controls
> data storage
> while fsio controls data flows. Memcg already handles too much, I just don't
> want add
> yet another unrelated stuff into it. Otherwise we will end with one single

I don't think handling dirty pages is an unrelated stuff to memcg. See
problem met by others using memcg:
https://lists.linux-foundation.org/pipermail/containers/2013-June/032917.html
Memcg dirty/writeback accounting is also only antipasto, long term
work is memcg aware dirty throttling and possibly per-memcg flusher.
Greg has already done some works here.


> controller
> which would handle all possible resources, because they all related in some
> cases.
>
>
>>
>> What's more, counting dirty/writeback stats in per-node manner can
>> bring inaccuracy in some situations: considering two tasks from
>> different fsio cgroups are dirtying one file concurrently but may only
>> be counting in one fsio stats, or a task is moved to another fsio
>> cgroup after dirtrying one file. As talking about task moving, it is
>> the root cause of adding memcg locks in page stat routines, since
>> there's a race window between 'modify cgroup owner' and 'update stats
>> using cgroup pointer'. But if you are going to handle task move or
>> take care of ->i_fsio for better accuracy in future, I'm afraid you
>> will also need some synchronization mechanism in hot paths. Maybe also
>> a new lock or mapping->tree_lock(which is already hot enough) IMHO.
>
>
> Yes, per-inode accounting is less accurate. But this approach works really
> well in the real life. I don't want add new locks and loose performance just
> to fix accuracy for some artificial cases.
>
> to Tejun:
> BTW I don't like that volatility of task's cgroup ponters. I'd like to
> forbid
> moving tasks between cgroups except for 'current', existing behavior can be
> kept with help of task_work: instead external change of task->cgroups we can
> schedule task_work into it in and change that pointer in the 'current'
> context.
> That will save us a lot of rcu_lock/unlock and atomic operations in grabbing
> temporary pointers to current cgroup because current->cgroups will be
> stable.
> I don't think that external cross-cgroup task migration is really
> performance
> critical. Currently I don't know what to do with kernel threads and
> workqueues,
> but any way this problem doesn't look unsolvable.
>



--
Thanks,
Sha

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