Re: [PATCH v2 6/6] blkcg, memcg: make blkcg depend on memcg on the default hierarchy

From: Vivek Goyal
Date: Wed Jul 09 2014 - 07:58:39 EST

On Tue, Jul 08, 2014 at 05:53:51PM -0400, Tejun Heo wrote:
> Hello, Vivek.
> On Tue, Jul 08, 2014 at 03:42:26PM -0400, Vivek Goyal wrote:
> > I have couple questions about new semantics. Following is my
> > understanding. Is it right?
> >
> > - So after this change one can not use blkio controller on unified
> > hiearchy if memory controller is mounted on some other hierarchy
> > and is not available for mounting unified hiearchy.
> Hmmm? No, the only behavior which changes is when both blkcg and
> memcg are mounted on the unified hierarchy. Nothing else changes.
> The dependency behavior kicks in iff memcg is available on the unified
> hierarchy.

Ok, good to know that dependency kicks in only if controlle being depended
on is available on the hierarchy.
> > - If blkio controller is enabled on unified hiearchy (memory controller
> > implicitly enabled), then one can't mount memory controller on other
> > hierarchies without first disabling blkio controller on unified hiearchy.
> Yes, blkio needs to be disabled to the root for memcg to be able to
> become free. This is an extra restriction but I don't think it's
> anything drastic. Once a controller starts being actively used on any
> hierarchy, nothing has been guaranteed about when the controller would
> become free again even if the whole hierarchy is reduced to the root.

Agreed. Thanks for the clarification.

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