Messages in this thread | | | Date | Fri, 17 Feb 2012 17:29:09 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH 7/9] block: implement bio_associate_current() |
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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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