Subject: Re: [PATCH net-next 2/9] bonding: remove bond read lock for bond_mii_monitor()

From: Ding Tianhong <[email-masked]>
Date: Thu, 7 Nov 2013 09:38:36 +0800

On 2013/11/7 7:18, Jay Vosburgh wrote:
> Ding Tianhong <[email-masked]> wrote:
> 
>> The bond_mii_monitor() still use bond lock read to protect
>> bond_has_slaves() and bond_miimon_inspect(), it is no effect,
>> so I move the RTNL to the top of the function to protect the
>> whole monitor, of course, if the bond state did not changed,
>> the monitor still calling RTNL, it may bring more performance
>> loss, but as a slow path, it is negligible.
> 
> 	I'm not sure this last part is true (about it being ok to
> acquire RTNL every pass).  The reason the bond_miimon_* functions are
> arranged in the way they are is specifically to avoid taking RTNL
> unnecessarily.  A common setting is miimon=100, which will acquire and
> release RTNL ten times per second.
> 
> 	The inspect function can be make RCU safe, and then the function
> will operate pretty much as it does now (with the multiple phases).  If
> a slave disappears between the phases, that's ok; one extra cycle on
> RTNL isn't a big deal, but 10 per second arguably is.
> 
> 	My comment also applies to the later patches in the series that
> make similar "always acquire RTNL" changes to the ARP monitor, ALB
> monitoring function, and the 802.3ad state machine.  That would be
> patches:
> 
> Subject: [PATCH net-next 3/9] bonding: rebuild the lock use for
>  bond_alb_monitor()
> Subject: [PATCH net-next 4/9] bonding: rebuild the lock use for
>  bond_loadbalance_arp_mon()
> Subject: [PATCH net-next 5/9] bonding: rebuild the lock use for
>  bond_activebackup_arp_mon()
> Subject: [PATCH net-next 6/9] bonding: use RTNL instead of bond lock for
>  bond_3ad_state_machine_handler()
> 
> 	The 802.3ad state machine patch or the balance-alb patch,
> combined with the miimon patch, will acquire and release the RTNL lock
> 20 times per second (10 for the 3ad state machine or alb monitor, and 10
> more for a typical miimon configuration).  I don't believe this is a
> reasonable implementation.
> 
> 	-J
> 

Thansk for your reply. your opinion is reasonable and clear, I will
optimization the lock for these patch, the RTNL should be replace by 
RCU in some place.

for better performance :)

Ding

>> also in bond_miimon_commit(), I remove the unwanted curr_slave_lock
>> when calling the bond_select_active_slave(), the RTNL is enough.
>>
>> Signed-off-by: Ding Tianhong <[email-masked]>
>> ---
>> drivers/net/bonding/bond_main.c | 44 ++++++++++++-----------------------------
>> 1 file changed, 13 insertions(+), 31 deletions(-)
>>
>> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>> index 9c9803c..98171eb 100644
>> --- a/drivers/net/bonding/bond_main.c
>> +++ b/drivers/net/bonding/bond_main.c
>> @@ -2074,9 +2074,7 @@ static void bond_miimon_commit(struct bonding *bond)
>> do_failover:
>> 		ASSERT_RTNL();
>> 		block_netpoll_tx();
>> -		write_lock_bh(&bond->curr_slave_lock);
>> 		bond_select_active_slave(bond);
>> -		write_unlock_bh(&bond->curr_slave_lock);
>> 		unblock_netpoll_tx();
>> 	}
>>
>> @@ -2098,47 +2096,31 @@ void bond_mii_monitor(struct work_struct *work)
>> 	bool should_notify_peers = false;
>> 	unsigned long delay;
>>
>> -	read_lock(&bond->lock);
>> -
>> 	delay = msecs_to_jiffies(bond->params.miimon);
>>
>> -	if (!bond_has_slaves(bond))
>> +	if (!rtnl_trylock()) {
>> +		delay = 1;
>> 		goto re_arm;
>> +	}
>>
>> -	should_notify_peers = bond_should_notify_peers(bond);
>> -
>> -	if (bond_miimon_inspect(bond)) {
>> -		read_unlock(&bond->lock);
>> -
>> -		/* Race avoidance with bond_close cancel of workqueue */
>> -		if (!rtnl_trylock()) {
>> -			read_lock(&bond->lock);
>> -			delay = 1;
>> -			should_notify_peers = false;
>> -			goto re_arm;
>> -		}
>> +	if (!bond_has_slaves(bond)) {
>> +		rtnl_unlock();
>> +		goto re_arm;
>> +	}
>>
>> -		read_lock(&bond->lock);
>> +	should_notify_peers = bond_should_notify_peers(bond);
>>
>> +	if (bond_miimon_inspect(bond))
>> 		bond_miimon_commit(bond);
>>
>> -		read_unlock(&bond->lock);
>> -		rtnl_unlock();	/* might sleep, hold no other locks */
>> -		read_lock(&bond->lock);
>> -	}
>> +	if (should_notify_peers)
>> +		call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, bond->dev);
>> +
>> +	rtnl_unlock();
>>
>> re_arm:
>> 	if (bond->params.miimon)
>> 		queue_delayed_work(bond->wq, &bond->mii_work, delay);
>> -
>> -	read_unlock(&bond->lock);
>> -
>> -	if (should_notify_peers) {
>> -		if (!rtnl_trylock())
>> -			return;
>> -		call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, bond->dev);
>> -		rtnl_unlock();
>> -	}
>> }
>>
>> static bool bond_has_this_ip(struct bonding *bond, __be32 ip)
>> -- 
>> 1.8.2.1
> 
> ---
> 	-Jay Vosburgh, IBM Linux Technology Center, [email-masked]
> 
> 
> .
>