Re: [PATCH 08/23] cgroup: pass around cgroup_subsys_state instead ofcgroup in subsystem methods

From: Vivek Goyal
Date: Mon Aug 05 2013 - 08:46:19 EST

On Thu, Aug 01, 2013 at 05:49:46PM -0400, Tejun Heo wrote:
> cgroup is currently in the process of transitioning to using struct
> cgroup_subsys_state * as the primary handle instead of struct cgroup *
> in subsystem implementations for the following reasons.
> * With unified hierarchy, subsystems will be dynamically bound and
> unbound from cgroups and thus css's (cgroup_subsys_state) may be
> created and destroyed dynamically over the lifetime of a cgroup,
> which is different from the current state where all css's are
> allocated and destroyed together with the associated cgroup. This
> in turn means that cgroup_css() should be synchronized and may
> return NULL, making it more cumbersome to use.
> * Differing levels of per-subsystem granularity in the unified
> hierarchy means that the task and descendant iterators should behave
> differently depending on the specific subsystem the iteration is
> being performed for.
> * In majority of the cases, subsystems only care about its part in the
> cgroup hierarchy - ie. the hierarchy of css's. Subsystem methods
> often obtain the matching css pointer from the cgroup and don't
> bother with the cgroup pointer itself. Passing around css fits
> much better.
> This patch converts all cgroup_subsys methods to take @css instead of
> @cgroup. The conversions are mostly straight-forward. A few
> noteworthy changes are
> * ->css_alloc() now takes css of the parent cgroup rather than the
> pointer to the new cgroup as the css for the new cgroup doesn't
> exist yet. Knowing the parent css is enough for all the existing
> subsystems.
> * In kernel/cgroup.c::offline_css(), unnecessary open coded css
> dereference is replaced with local variable access.
> This patch shouldn't cause any behavior differences.
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> Cc: Aristeu Rozanski <aris@xxxxxxxxxx>
> Cc: Matt Helsley <matthltc@xxxxxxxxxx>
> Cc: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
> block/blk-cgroup.c | 25 +++++++++++-----------

blk-cgroup changes look good to me.

Acked-by: Vivek Goyal <vgoyal@xxxxxxxxxx>

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