Re: [PATCHSET cgroup/for-3.8] cpuset: decouple cpuset locking from cgroup core

[Kamezawa Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>]

(2012/11/29 6:34), Tejun Heo wrote:
> Hello, guys.
> 
> Depending on cgroup core locking - cgroup_mutex - is messy and makes
> cgroup prone to locking dependency problems.  The current code already
> has lock dependency loop - memcg nests get_online_cpus() inside
> cgroup_mutex.  cpuset the other way around.
> 
> Regardless of the locking details, whatever is protecting cgroup has
> inherently to be something outer to most other locking constructs.
> cgroup calls into a lot of major subsystems which in turn have to
> perform subsystem-specific locking.  Trying to nest cgroup
> synchronization inside other locks isn't something which can work
> well.
> 
> cgroup now has enough API to allow subsystems to implement their own
> locking and cgroup_mutex is scheduled to be made private to cgroup
> core.  This patchset makes cpuset implement its own locking instead of
> relying on cgroup_mutex.
> 
> cpuset is rather nasty in this respect.  Some of it seems to have come
> from the implementation history - cgroup core grew out of cpuset - but
> big part stems from cpuset's need to migrate tasks to an ancestor
> cgroup when an hotunplug event makes a cpuset empty (w/o any cpu or
> memory).
> 
> This patchset decouples cpuset locking from cgroup_mutex.  After the
> patchset, cpuset uses cpuset-specific cpuset_mutex instead of
> cgroup_mutex.  This also removes the lockdep warning triggered during
> cpu offlining (see 0009).
> 
> Note that this leaves memcg as the only external user of cgroup_mutex.
> Michal, Kame, can you guys please convert memcg to use its own locking
> too?
> 

Hmm. let me see....at quick glance cgroup_lock() is used at
  hierarchy policy change
  kmem_limit
  migration policy change
  swapiness change
  oom control

Because all aboves takes care of changes in hierarchy,
Having a new memcg's mutex in ->create() may be a way.

Ah, hm, Costa is mentioning task-attach. is the task-attach problem in memcg ?

Thanks,
-Kame











> This patchset contains the following thirteen patches.
> 
>   0001-cpuset-remove-unused-cpuset_unlock.patch
>   0002-cpuset-remove-fast-exit-path-from-remove_tasks_in_em.patch
>   0003-cpuset-introduce-css_on-offline.patch
>   0004-cpuset-introduce-CS_ONLINE.patch
>   0005-cpuset-introduce-cpuset_for_each_child.patch
>   0006-cpuset-cleanup-cpuset-_can-_attach.patch
>   0007-cpuset-drop-async_rebuild_sched_domains.patch
>   0008-cpuset-reorganize-CPU-memory-hotplug-handling.patch
>   0009-cpuset-don-t-nest-cgroup_mutex-inside-get_online_cpu.patch
>   0010-cpuset-make-CPU-memory-hotplug-propagation-asynchron.patch
>   0011-cpuset-pin-down-cpus-and-mems-while-a-task-is-being-.patch
>   0012-cpuset-schedule-hotplug-propagation-from-cpuset_atta.patch
>   0013-cpuset-replace-cgroup_mutex-locking-with-cpuset-inte.patch
> 
> 0001-0006 are prep patches.
> 
> 0007-0009 make cpuset nest get_online_cpus() inside cgroup_mutex, not
> the other way around.
> 
> 0010-0012 plug holes which would be exposed by switching to
> cpuset-specific locking.
> 
> 0013 replaces cgroup_mutex with cpuset_mutex.
> 
> This patchset is on top of cgroup/for-3.8 (fddfb02ad0) and also
> available in the following git branch.
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-cpuset-locking
> 
> diffstat follows.
> 
>   kernel/cpuset.c |  750 +++++++++++++++++++++++++++++++-------------------------
>   1 file changed, 423 insertions(+), 327 deletions(-)
> 
> Thanks.
> 
> --
> tejun
> 
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