[WIP BRANCH] cgroups support in bfq-mq WIP branch

[Paolo Valente]

Hi,
I've just completed cgroups support, and I'd like to highlight the
main blk-mq issue that I have found along the way.  I have pushed the
commit that completes the support for cgroups to the usual WIP branch
[1].  Before moving to this issue, I have preliminary question about
the scheduler name, since I'm about to start preparing the patch
series for submission.  So far, I have used bfq-mq as a temporary
name.  Are we fine with it, or should I change it, for example, to
just bfq?  Jens?

I've found a sort of circular dependency in blk-mq, related to
scheduler initialization.  To describe both the issue and how I've
addressed it, I'm pasting the message of the new commit.

    This commit completes cgroups support for bfq-mq. In particular, it deals with
    a sort of circular dependency introduced in blk-mq: the function
    blkcg_activate_policy, invoked during scheduler initialization, triggers the
    invocation of the has_work scheduler hook (before the init function is
    finished). To adress this issue, this commit moves the invocation of
    blkcg_activate_policy after the initialization of all the fields that could be
    initialized before invoking blkcg_activate_policy itself. This enables has_work
    to correctly return false, and thus to prevent the blk-mq stack from invoking
    further scheduler hooks before the init function is finished.

Thanks,
Paolo

[1] https://github.com/Algodev-github/bfq-mq