On Tue, Nov 23, 2010 at 12:14 AM, Li Zefan <lizf@xxxxxxxxxxxxxx> wrote: > 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 believe the same comment applies as sched.c, I'll update the commit message. > I don't think the performance of task attaching is so critically > important that we have to use call_rcu() instead of synchronize_rcu()? On my desktop, moving a task between cgroups averages 100 ms, and on an Tegra2 SMP ARM platform it takes 20 ms. Moving a task with many threads can take hundreds of milliseconds or more. With this patch it takes 50 microseconds to move one task, a 400x improvement. >> 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? Left over from the previous patch that incorrectly dropped RCU completely. I'll put the rcu_read_locks back. >> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx> >> >> --- >> >> 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(-) > _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
