netdev - Re: [rfc] IPVS: convert scheduler management to RCU

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Date:	Fri, 20 Aug 2010 11:04:02 -0400
From:	yao zhao <dragonlinux@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Changli Gao <xiaosuo@...il.com>, Simon Horman <horms@...ge.net.au>,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	Stephen Hemminger <shemminger@...tta.com>,
	Wensong Zhang <wensong@...ux-vs.org>,
	Julian Anastasov <ja@....bg>,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [rfc] IPVS: convert scheduler management to RCU

On Fri, Aug 20, 2010 at 10:32 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 20 août 2010 à 10:16 -0400, yao zhao a écrit :
>
>> if it is not performance critical, you should use  the
>> read_lock/write_lock, it should make the readers happier than
>> spinlock. the name "mutex" is a little bit confuse.
>
> Yes, I mentioned the 'mutex' name oddity.
>
> Point is :
>
> We want to remove read_write locks. They dont fit the bill.
>
> If performance critical, lot of readers -> RCU (a lot faster)
> If not, or too much writers versus readers -> spinlock (a bit faster)
>
>
for writers more than readers of course spin will be better, that is
what read/write lock for and spin for.
But the case here is whether these register_ip_vs_scheduler or
unregister are more frequent than the readers.
if not definitely read_lock will better than spin_lock. although worse than rcu.
>
>> synchronize_rcu() is not necessary when you only need to delete from a
>> list as it is atomic.
>>
>
> Thats a rather strange and completely wrong claim. A big part of RCU job
> is to have appropriate work done on deletes. Inserts are more easy (only
> needs a smp_wmb())
>
> Take a look at Documentation/RCU/* before saying such things ;)
>
> Not only synchronize_rcu() is not enough to protect this kind of code,
> but you need something stronger.
>
>
>
The code here is deleting a global from the list, am I right? I didn't
see any called case.
what are you going to do more? free it? write_unlock_bh should make the mb.

yao
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