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    SubjectRe: [PATCH] cgroup: Remove call to synchronize_rcu in cgroup_attach_task
    On 11/23/2010 05:43 PM, Colin Cross wrote:
    > synchronize_rcu can be very expensive, averaging 100 ms in
    > some cases. In cgroup_attach_task, it is used to prevent
    > a task->cgroups pointer dereferenced in an RCU read side
    > critical section from being invalidated by delaying the call
    > to put_css_set until after an RCU grace period.
    > To avoid the call to synchronize_rcu, make the put_css_set
    > call rcu-safe by moving the deletion of the css_set links
    > into rcu-protected free_css_set_rcu.
    > The calls to check_for_release in free_css_set_rcu now occur
    > in softirq context, so convert all uses of the
    > release_list_lock spinlock to irq safe versions.
    > The decrement of the cgroup refcount is no longer
    > synchronous with the call to put_css_set, which can result
    > in the cgroup refcount staying positive after the last call
    > to cgroup_attach_task returns. To allow the cgroup to be
    > deleted with cgroup_rmdir synchronously after
    > cgroup_attach_task, introduce a second refcount,
    > rmdir_count, that is decremented synchronously in
    > put_css_set. If cgroup_rmdir is called on a cgroup for
    > hich rmdir_count is zero but count is nonzero, reuse the
    > rmdir waitqueue to block the rmdir until the rcu callback
    > is called.
    > Signed-off-by: Colin Cross <>
    > ---
    > This patch is similar to what you described. The main differences are
    > that I used a new atomic to handle the rmdir case, and I converted
    > check_for_release to be callable in softirq context rather than schedule
    > work in free_css_set_rcu. Your css_set scanning in rmdir sounds better,
    > I'll take another look at that. Is there any problem with disabling irqs
    > with spin_lock_irqsave in check_for_release?
    > include/linux/cgroup.h | 6 ++
    > kernel/cgroup.c | 124 ++++++++++++++++++++++++++++--------------------
    > 2 files changed, 78 insertions(+), 52 deletions(-)

    Colin, what became of this patch? I see this in your Tegra tree for

    I looked in linux-next but didn't see it there. This resolves a
    performance issue on MSM SMP so I'm curious if this is going upstream.

    - Bryan

    Sent by an employee of the Qualcomm Innovation Center, Inc.
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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