netdev - Re: [PATCH net 1/2] ipv4: avoid quadratic behavior in netns dismantle

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Date:   Tue, 18 Jan 2022 19:40:38 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     David Ahern <dsahern@...nel.org>, netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net 1/2] ipv4: avoid quadratic behavior in netns dismantle

On 1/18/22 1:46 PM, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> net/ipv4/fib_semantics.c uses an hash table of 256 slots,
> keyed by device ifindexes: fib_info_devhash[DEVINDEX_HASHSIZE]
> 
> Problem is that with network namespaces, devices tend
> to use the same ifindex.
> 
> lo device for instance has a fixed ifindex of one,
> for all network namespaces.
> 
> This means that hosts with thousands of netns spend
> a lot of time looking at some hash buckets with thousands
> of elements, notably at netns dismantle.
> 
> Simply add a per netns perturbation (net_hash_mix())
> to spread elements more uniformely.
> 
> Also change fib_devindex_hashfn() to use more entropy.
> 
> Fixes: aa79e66eee5d ("net: Make ifindex generation per-net namespace")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/fib_semantics.c | 36 +++++++++++++++++-------------------
>  1 file changed, 17 insertions(+), 19 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@...nel.org>