[Devel] Re: RFC: Attaching threads to cgroups is OK?

[Takuya Yoshikawa]

Hi both,

Fernando is off this week, so may not be able to reply soon, sorry.

Vivek Goyal wrote:
> On Fri, Sep 05, 2008 at 09:00:17PM +0900, Hirokazu Takahashi wrote:
>> Hi, fernando,
>>
>>>> IMHO, optimizing the synchronous path alone would justify the addition
>>>> of io_context in bio. There is more to this though.
>>>>
>>>> As you point out, it would seem that aio and buffered IO would not
>>>> benefit from caching the io context in the bio itself, but there are
>>>> some subtleties here. Let's consider stacking devices and buffered IO,
>>>> for example. When a bio enters such a device it may get replicated
>>>> several times and, depending on the topology, some other derivative bios
>>>> will be created (RAID1 and parity configurations come to mind,
>>>> respectively). The problem here is that the memory allocated for the
>>>> newly created bios will be owned by the corresponding dm or md kernel
>>>> thread, not the originator of the bio we are replicating or calculating
>>>> the parity bits from.
>>> I've already tried implementing this feature. Will you take a look
>>> at the thread whose subject is "I/O context inheritance" in
>>> http://www.uwsg.iu.edu/hypermail/linux/kernel/0804.2/index.html#2857.
>> When I started to implement this, I would make each bio have two
>> io_contexts -- per-process io_context which had ionice and
>> and per-cgroup io_context.
>>

I am also trying to make bios have pointers to io_contexts and I have a
question concerning this, very simple one.

Is it OK to think bio->bi_io_context is a cache to find the io_context
of an appropriate task, an issuer of this bio, which may sometimes not be
the direct one, e.g. stacking devices case.

If so, is it same if we make bios have pointers to the issuers, tasks or
cgroups, of them and then find the io_contexts through these?

I sometimes tempted to say "this bio's io_context."

> 
> Hi Hirokazu,
> 
> I had a question. Why are we trying to create another io_context or why
> are we trying to mix up existing io_context (which is per task or per
> thread group) with cgroups?
> 
> To me we just need to know the "cgroup id" to take a specific action with
> a bio. We don't need whole io_context strucutre. So can't we just use
> something like, page->page_cgroup->bio_cgroup->cgrou_id or something like
> that.
> 
> What I mean is that the only thing which we seem to require to differentiate
> between various bio is cgroup id it belongs to and that can be a single
> "unsigned long" stored at appropriate place. Why are we looking at
> creating a full io_context structure and trying to share it among all the
> members of a cgroup?

> 
> Thanks
> Vivek

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