Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support

[Paolo Valente]

Il giorno 22/apr/2016, alle ore 20:41, Tejun Heo <tj@xxxxxxxxxx> ha scritto:

> Hello, Paolo.
> 
> On Fri, Apr 22, 2016 at 08:19:47PM +0200, Paolo Valente wrote:
>>> So, a kworker would jump through different workqueues and issue IOs
>>> for different writeback domains and the context can't be tied to the
>>> issuing task.  The cgroup membership should be determined directly
>>> from the bio.
>> 
>> Yes. My doubt arises from the fact that the only source of intense I/O
>> is the dd (I have executed it alone). In contrast, group changes occur
>> at a high frequency during all the execution of the dd. Apparently I
>> cannot see any other I/O induced by the dd. Journaling issues sync
>> requests.
>> 
>>> cfq uses per-cgroup async queue.  I'm not sure how this
>>> would map to bfq tho.
>> 
>> It’s the same. But this is the part I’m checking.
> 
> Ah, right, I was confused.  cic is always associated with the task and
> yes a writeback worker can trigger blkcg changed events frequently as
> it walks through different cgroups.  Is this an issue?
> 

That’s exactly the source of my confusion: why does the worker walk through different cgroups all the time if the I/O is originated by the same process, which never changes group?

Thanks,
Paolo

> Thanks.
> 
> -- 
> tejun