Messages in this thread | | | Date | Tue, 23 Nov 2010 16:14:24 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] cgroup: Convert synchronize_rcu to call_rcu in cgroup_attach_task |
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12:06, Colin Cross wrote: > The synchronize_rcu call in cgroup_attach_task can be very > expensive. All fastpath accesses to task->cgroups that expect > task->cgroups not to change already use task_lock() or > cgroup_lock() to protect against updates, and, in cgroup.c, > only the CGROUP_DEBUG files have RCU read-side critical > sections. > > sched.c uses RCU read-side-critical sections on task->cgroups, > but only to ensure that a dereference of task->cgroups does > not become invalid, not that it doesn't change. >
Other cgroup subsystems also use rcu_read_lock to access task->cgroups, for example net_cls cgroup and device cgroup.
I don't think the performance of task attaching is so critically important that we have to use call_rcu() instead of synchronize_rcu()?
> This patch adds a function put_css_set_rcu, which delays the > put until after a grace period has elapsed. This ensures that > any RCU read-side critical sections that dereferenced > task->cgroups in sched.c have completed before the css_set is > deleted. The synchronize_rcu()/put_css_set() combo in > cgroup_attach_task() can then be replaced with > put_css_set_rcu(). >
> Also converts the CGROUP_DEBUG files that access > current->cgroups to use task_lock(current) instead of > rcu_read_lock(). >
What for? What do we gain from doing this for those debug interfaces?
> Signed-off-by: Colin Cross <ccross@android.com> > > --- > > This version fixes the problems with the previous patch by > keeping the use of RCU in cgroup_attach_task, but allowing > cgroup_attach_task to return immediately by deferring the > final put_css_reg to an rcu callback. > > include/linux/cgroup.h | 4 +++ > kernel/cgroup.c | 58 ++++++++++++++++++++++++++++++++++++++---------- > 2 files changed, 50 insertions(+), 12 deletions(-)
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