[Devel] Re: [PATCH] cgroup: Remove call to synchronize_rcu in cgroup_attach_task

[Bryan Huntsman]

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 <ccross at android.com>
> ---
> 
> 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
Android.

http://android.git.kernel.org/?p=kernel/tegra.git;a=commit;h=05946a1e0fdb011ac0e6638ee60b181c584f127b

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.
 Thanks.

- Bryan


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The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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