netdev - Re: [PATCH net] net: reorder 'struct net' fields to avoid false sharing

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Date:   Sat, 19 Oct 2019 12:22:35 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        oliver.sang@...el.com
Subject: Re: [PATCH net] net: reorder 'struct net' fields to avoid false
 sharing

From: Eric Dumazet <edumazet@...gle.com>
Date: Fri, 18 Oct 2019 15:20:05 -0700

> Intel test robot reported a ~7% regression on TCP_CRR tests
> that they bisected to the cited commit.
> 
> Indeed, every time a new TCP socket is created or deleted,
> the atomic counter net->count is touched (via get_net(net)
> and put_net(net) calls)
> 
> So cpus might have to reload a contended cache line in
> net_hash_mix(net) calls.
> 
> We need to reorder 'struct net' fields to move @hash_mix
> in a read mostly cache line.
> 
> We move in the first cache line fields that can be
> dirtied often.
> 
> We probably will have to address in a followup patch
> the __randomize_layout that was added in linux-4.13,
> since this might break our placement choices.
> 
> Fixes: 355b98553789 ("netns: provide pure entropy for net_hash_mix()")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: kernel test robot <oliver.sang@...el.com>

Applied and queued up for -stable, thanks Eric.