netdev - Re: [RFC net-next] net ipv6: convert fib6

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Date:   Mon, 31 Jul 2017 19:57:04 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, Kernel-team@...com,
        Shaohua Li <shli@...com>, Wei Wang <weiwan@...gle.com>
Subject: Re: [RFC net-next] net ipv6: convert fib6_table rwlock to a percpu
 lock

On Mon, Jul 31, 2017 at 04:10:07PM -0700, Stephen Hemminger wrote:
> On Mon, 31 Jul 2017 10:18:57 -0700
> Shaohua Li <shli@...nel.org> wrote:
> 
> > From: Shaohua Li <shli@...com>
> > 
> > In a syn flooding test, the fib6_table rwlock is a significant
> > bottleneck. While converting the rwlock to rcu sounds straighforward,
> > but is very challenging if it's possible. A percpu spinlock is quite
> > trival for this problem since updating the routing table is a rare
> > event. In my test, the server receives around 1.5 Mpps in syn flooding
> > test without the patch in a dual sockets and 56-CPU system. With the
> > patch, the server receives around 3.8Mpps, and perf report doesn't show
> > the locking issue.
> > 
> > Cc: Wei Wang <weiwan@...gle.com>
> 
> You just reinvented brlock...

you mean lglock? It has been removed from kernel.
 
> RCU is not that hard, why not do it right?

Maybe. But don't think it's the reason why we shouldn't do the percpu lock now,
this is a simple change, if some smart guys find a way of RCU, we can easily
remove this.