linux-kernel - Re: [PATCH v2 08/23] cgroup: pass around cgroup_subsys_state instead of cgroup in subsystem methods

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Message-ID: <5200A391.9030208@bmw-carit.de>
Date:	Tue, 6 Aug 2013 09:19:45 +0200
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	Tejun Heo <tj@...nel.org>
CC:	<lizefan@...wei.com>, <containers@...ts.linux-foundation.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	Aristeu Rozanski <aris@...hat.com>,
	Matt Helsley <matthltc@...ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 08/23] cgroup: pass around cgroup_subsys_state instead
 of cgroup in subsystem methods

On 08/02/2013 10:24 PM, 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.
>
> v2: Unnecessary explicit cgrp->subsys[] deref in css_online() replaced
>      with local variable @css as suggested by Li Zefan.
>
>      Rebased on top of new for-3.12 which includes for-3.11-fixes so
>      that ->css_free() invocation added by da0a12caff ("cgroup: fix a
>      leak when percpu_ref_init() fails") is converted too.  Suggested
>      by Li Zefan.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Acked-by: Li Zefan <lizefan@...wei.com>
> Acked-by: Michal Hocko <mhocko@...e.cz>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Balbir Singh <bsingharora@...il.com>
> Cc: Aristeu Rozanski <aris@...hat.com>
> Cc: Matt Helsley <matthltc@...ibm.com>
> Cc: Daniel Wagner <daniel.wagner@...-carit.de>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Steven Rostedt <rostedt@...dmis.org>

netprio and cls part:

Acked-By: Daniel Wagner <daniel.wagner@...-carit.de>

cheers,
daniel



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