Re: [PATCH 15/20] io-controller: map async requests to appropriate cgroup

Nauman Rafique
2009-05-28T18:05:04Z

On Thu, May 28, 2009 at 9:57 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Thu, May 28, 2009 at 06:27:40PM +0900, Ryo Tsuruta wrote:
>> Hi Vivek,
>>
>> > +#ifdef CONFIG_TRACK_ASYNC_CONTEXT
>> > +   if (elv_bio_sync(bio)) {
>> > +           /* sync io. Determine cgroup from submitting task context. */
>> > +           cgroup = task_cgroup(current, io_subsys_id);
>> > +           return cgroup;
>> > +   }
>> > +
>> > +   /* Async io. Determine cgroup from with cgroup id stored in page */
>> > +   bio_cgroup_id = get_blkio_cgroup_id(bio);
>> > +
>> > +   if (!bio_cgroup_id)
>> > +           return NULL;
>> > +
>> > +   cgroup = blkio_cgroup_lookup(bio_cgroup_id);
>> > +#else
>> > +   cgroup = task_cgroup(current, io_subsys_id);
>> > +#endif
>> > +   return cgroup;
>> > +}
>>
>> There is a case where a kernel thread (such as device-mapper drivers)
>> submits a sync IO instead of a task which originates the IO. I think
>> you should always use get_blkio_cgroup_id() to determine cgroup.
>>
>
> Hi Ryo,
>
> Ok. Can you give some examples of drivers which are submitting reads in
> different context al-together. You mentioned in the past that dm-crypt
> looks like the one. How does current CFQ takes care of that. So if a
> BE prio 7 or an RT prio 0, task is submitting a READ, CFQ will not know it
> and it will put that READ in the queue of the READ submitting device
> mapper thread (may be BE prio 3 or 4)?
>
> Always determining the cgroup from bio, will make things slower at the
> same time complicated from the CFQ point of view. Right now cfq creates
> and caches the queue pointer in the io context of the bio submitting task
> and assumes sync requests are coming from that task/io context. Currently
> there can only be one sync queue associated with one context. So if a single
> thread is submitting reads (may be a worker thread) on behalf of other
> processes, then we loose the io context information. In fact currently we
> don't even carry ioprio and io class information in bio.
>
> So looks like we need to carry task io context information also in bio
> to be able to associate the bio to right queue at CFQ level. This makes
> it bit more complicated. For the time being I will keep it in my TODO
> list and handle it once other more severe problems have been taken care
> of.

>From what I remember, Fernando pointed out the same problem in LFS'09
(device mapper drivers submitting sync bio) for his motivation to
track io_context in bio. Fernando, what is the state the patch you
were working on?

>
> Thanks
> Vivek
>