Re: Possible netns creation and execution performance/scalability regression since v3.8 due to rcu callbacks being offloaded to multiple cpus

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jun 11, 2014 at 04:12:15PM -0700, Eric W. Biederman wrote:
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> 
> > On Wed, Jun 11, 2014 at 01:46:08PM -0700, Eric W. Biederman wrote:
> >> On the chance it is dropping the old nsproxy which calls syncrhonize_rcu
> >> in switch_task_namespaces that is causing you problems I have attached
> >> a patch that changes from rcu_read_lock to task_lock for code that
> >> calls task_nsproxy from a different task.  The code should be safe
> >> and it should be an unquestions performance improvement but I have only
> >> compile tested it.
> >> 
> >> If you can try the patch it will tell is if the problem is the rcu
> >> access in switch_task_namespaces (the only one I am aware of network
> >> namespace creation) or if the problem rcu case is somewhere else.
> >> 
> >> If nothing else knowing which rcu accesses are causing the slow down
> >> seem important at the end of the day.
> >> 
> >> Eric
> >> 
> >
> > If this is the culprit, another approach would be to use workqueues from
> > RCU callbacks.  The following (untested, probably does not even build)
> > patch illustrates one such approach.
> 
> For reference the only reason we are using rcu_lock today for nsproxy is
> an old lock ordering problem that does not exist anymore.
> 
> I can say that in some workloads setns is a bit heavy today because of
> the synchronize_rcu and setns is more important that I had previously
> thought because pthreads break the classic unix ability to do things in
> your process after fork() (sigh).
> 
> Today daemonize is gone, and notify the parent process with a signal
> relies on task_active_pid_ns which does not use nsproxy.  So the old
> lock ordering problem/race is gone.
> 
> The description of what was happening when the code switched from
> task_lock to rcu_read_lock to protect nsproxy.

OK, never mind, then!  ;-)

							Thanx, Paul

> commit cf7b708c8d1d7a27736771bcf4c457b332b0f818
> Author: Pavel Emelyanov <xemul@openvz.org>
> Date:   Thu Oct 18 23:39:54 2007 -0700
> 
>     Make access to task's nsproxy lighter
>     
>     When someone wants to deal with some other taks's namespaces it has to lock
>     the task and then to get the desired namespace if the one exists.  This is
>     slow on read-only paths and may be impossible in some cases.
>     
>     E.g.  Oleg recently noticed a race between unshare() and the (sent for
>     review in cgroups) pid namespaces - when the task notifies the parent it
>     has to know the parent's namespace, but taking the task_lock() is
>     impossible there - the code is under write locked tasklist lock.
>     
>     On the other hand switching the namespace on task (daemonize) and releasing
>     the namespace (after the last task exit) is rather rare operation and we
>     can sacrifice its speed to solve the issues above.
>     
>     The access to other task namespaces is proposed to be performed
>     like this:
>     
>          rcu_read_lock();
>          nsproxy = task_nsproxy(tsk);
>          if (nsproxy != NULL) {
>                  / *
>                    * work with the namespaces here
>                    * e.g. get the reference on one of them
>                    * /
>          } / *
>              * NULL task_nsproxy() means that this task is
>              * almost dead (zombie)
>              * /
>          rcu_read_unlock();
>     
>     This patch has passed the review by Eric and Oleg :) and,
>     of course, tested.
>     
>     [clg@fr.ibm.com: fix unshare()]
>     [ebiederm@xmission.com: Update get_net_ns_by_pid]
>     Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
>     Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>     Cc: Oleg Nesterov <oleg@tv-sign.ru>
>     Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>     Cc: Serge Hallyn <serue@us.ibm.com>
>     Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
>     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> 
> Eric
>