RFC: Attaching threads to cgroups is OK?

Balbir Singh balbir at linux.vnet.ibm.com
Wed Aug 20 20:32:04 PDT 2008

Fernando Luis Vázquez Cao wrote:
> On Wed, 2008-08-20 at 20:48 +0900, Hirokazu Takahashi wrote:
>> Hi,
>>>> Tsuruta-san, how about your bio-cgroup's tracking concerning this?
>>>> If we want to use your tracking functions for each threads seperately, 
>>>> there seems to be a problem.
>>>> ===cf. mm_get_bio_cgroup()===================
>>>>            owner
>>>> mm_struct ----> task_struct ----> bio_cgroup
>>>> =============================================
>>>> In my understanding, the mm_struct of a thread is same as its parent's.
>>>> So, even if we attach the TIDs of some threads to different cgroups the 
>>>> tracking always returns the same bio_cgroup -- its parent's group.
>>>> Do you have some policy about in which case we can use your tracking?
>>> It's will be resitriction when io-controller reuse information of the owner
>>> of memory. But if it's very clear who issues I/O (by tracking read/write
>>> syscall), we may have chance to record the issuer of I/O to page_cgroup
>>> struct. 
>> This might be slightly different topic though,
>> I've been thinking where we should add hooks to track I/O reqeust.
>> I think the following set of hooks is enough whether we are going to
>> support thread based cgroup or not.
>>   Hook-1: called when allocating a page, where the memory controller
>> 	  already have a hoook.
>>   Hook-2: called when making a page in page-cache dirty.
>> For anonymous pages, Hook-1 is enough to track any type of I/O request.
>> For pages in page-cache, Hook-1 is also enough for read I/O because
>> the I/O is issued just once right after allocting the page.
>> For write I/O requests to pages in page-cache, Hook-1 will be okay
>> in most cases but sometimes process in another cgroup may write
>> the pages. In this case, Hook-2 is needed to keep accurate to track
>> I/O requests.
> This relative simplicity is what prompted me to say that we probably
> should try to disentangle the io tracking functionality from the memory
> controller a bit more (of course we still should reuse as much as we can
> from it). The rationale for this is that the existing I/O scheduler
> would benefit from proper io tracking capabilities too, so it'd be nice
> if we could have them even in non-cgroup-capable kernels.

Hook 2 referred to in the mail above exist today in the form of task IO accounting.

> As an aside, when the IO context of a certain IO operation is known
> (synchronous IO comes to mind) I think it should be cashed in the
> resulting bio so that we can do without the expensive accesses to
> bio_cgroup once it enters the block layer.

Will this give you everything you need for accounting and control (from the
block layer?)


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