On Tue, Nov 23, 2010 at 12:58 AM, Colin Cross <ccross@xxxxxxxxxxx> wrote: > 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(-) >> > This patch has another problem - calling put_css_set_rcu twice before an rcu grace period has elapsed would not guarantee the appropriate rcu grace period for the second call. I'll try a new approach, moving the parts of put_css_set that need to be protected by rcu into free_css_set_rcu. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
