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SubjectRe: [PATCH 7/9] block: implement bio_associate_current()
On Fri, Feb 17, 2012 at 02:03:51PM -0800, Tejun Heo wrote:
> Hello, Vivek.
>
> On Fri, Feb 17, 2012 at 04:33:13PM -0500, Vivek Goyal wrote:
> > On Thu, Feb 16, 2012 at 02:37:56PM -0800, Tejun Heo wrote:
> >
> > [..]
> > > This patch implements bio_associate_current() which associates the
> > > specified bio with %current. The bio will record the associated ioc
> > > and blkcg at that point and block layer will use the recorded ones
> > > regardless of which task actually ends up issuing the bio. bio
> > > release puts the associated ioc and blkcg.
> >
> > How about storing blkcg information in io_context instead of bio. We will
> > have less copies of bio pointers and I think logically it makes sense.
>
> I don't know. The problem with that approach is that we introduce a
> persistent state which needs to be kept in sync. cgroup is a task
> property and the current code just grabs the current cgroup of
> %current and uses it for that bio. It doesn't matter how the task
> changes its cgroup membership later - we're correct (in a sense) no
> matter what. If we add cgroup pointer to ioc, we need to keep that in
> sync with task changing cgroup memberships and need to introduce
> synchronization scheme for accessing ioc->blkcg, which is a much
> bigger headache.

Don't we already keep track of task changing cgroup and record that
state in ioc.

blkiocg_attach()
ioc_cgroup_changed()

I think in ioc_cgroup_changed() we can just drop the reference to previous
blkcg and store reference to new blkcg?

>
> I think it's better to take an explicit ref now. If the situation
> changes, it's an implementation detail only known to block layer
> proper anyway, so we should be able to change it without too much
> difficulty.

I am fine with changing it later too.

BTW, this change seems to be completely orthogonal to blkcg cleanup. May
be it is a good idea to split it out in a separate patch series. It has
nothing to do with rcu cleanup in blkcg.

Thanks
Vivek


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