Re: [PATCH net-next-2.6] net: add dev_close_many

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 13 décembre 2010 à 16:18 +0200, Octavian Purdila a écrit :
> Add dev_close_many and dev_deactivate_many to factorize another
> expensive sync-rcu operation in the netdevice unregister path.
> 
> $ modprobe dummy numdummies=10000
> $ ip link set dev dummy* up
> $ time rmmod dummy
> 
> Without the patch           With the patch
> 
> real    0m 24.63s           real    0m 5.15s
> user    0m 0.00s            user    0m 0.00s
> sys     0m 6.05s            sys     0m 5.14s
> 
> Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
> ---

Hmm, I think this solves the "rmmod dummy" case, but not the "dismantle
devices one by one", which is the general one (on heavy duty tunnels/ppp
servers)

I think we could use a kernel thread (a workqueue presumably), handling
3 lists of devices to be dismantled, respecting one rcu grace period (or
rcu_barrier()) before transfert of one item from one list to following
one.

This way, each device removal could post a device to this kernel thread
and return to user immediately. Time of RTNL hold would be reduced
(calls to synchronize_rcu() would be done with RTNL not held)