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

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Message-ID: <1501525853.1876.22.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 31 Jul 2017 11:30:53 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Shaohua Li <shli@...nel.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, 2017-07-31 at 10:18 -0700, Shaohua Li 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.
> 

> +static inline void fib6_table_write_lock_bh(struct fib6_table *table)
> +{
> +	int i;
> +
> +	spin_lock_bh(per_cpu_ptr(table->percpu_tb6_lock, 0));
> +	for_each_possible_cpu(i) {
> +		if (i == 0)
> +			continue;
> +		spin_lock_nest_lock(per_cpu_ptr(table->percpu_tb6_lock, i),
> +			per_cpu_ptr(table->percpu_tb6_lock, 0));
> +	}
> +}

Your code assumes that cpu 0 is valid.

I would rather not hard code this knowledge.

Also this is not clear why you need the nested thing.