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    SubjectRe: [PATCH 2/2] cgroup: make css->refcnt clearing on cgroup removal optional
    From
    2012年3月31日7:36 Tejun Heo <tj@kernel.org>:
    > Currently, cgroup removal tries to drain all css references. If there
    > are active css references, the removal logic waits and retries
    > ->pre_detroy() until either all refs drop to zero or removal is
    > cancelled.
    >
    > This semantics is unusual and adds non-trivial complexity to cgroup
    > core and IMHO is fundamentally misguided in that it couples internal
    > implementation details (references to internal data structure) with
    > externally visible operation (rmdir). To userland, this is a behavior
    > peculiarity which is unnecessary and difficult to expect (css refs is
    > otherwise invisible from userland), and, to policy implementations,
    > this is an unnecessary restriction (e.g. blkcg wants to hold css refs
    > for caching purposes but can't as that becomes visible as rmdir hang).
    >
    > Unfortunately, memcg currently depends on ->pre_destroy() retrials and
    > cgroup removal vetoing and can't be immmediately switched to the new
    > behavior. This patch introduces the new behavior of not waiting for
    > css refs to drain and maintains the old behavior for subsystems which
    > have __DEPRECATED_clear_css_refs set.
    >
    > Once, memcg is updated, we can drop the code paths for the old
    > behavior as proposed in the following patch. Note that the following
    > patch is incorrect in that dput work item is in cgroup and may lose
    > some of dputs when multiples css's are released back-to-back, and
    > __css_put() triggers check_for_release() when refcnt reaches 0 instead
    > of 1; however, it shows what part can be removed.
    >
    > http://thread.gmane.org/gmane.linux.kernel.containers/22559/focus=75251
    >
    > Note that, in not-too-distant future, cgroup core will start emitting
    > warning messages for subsys which require the old behavior, so please
    > get moving.
    >
    Ok, I'll update memcg ASAP (after LSF/MM).
    I'm now thinking to move all resources to root if use_hierarchy=0.

    Thanks,
    -Kame


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