Subject: Re: [PATCH net] nexthop: Fix performance regression in nexthop deletion

From: Jesse Brandeburg <[email-masked]>
Date: Fri, 16 Oct 2020 14:46:36 -0700

Ido Schimmel wrote:

> From: Ido Schimmel <[email-masked]>
> 
> While insertion of 16k nexthops all using the same netdev ('dummy10')
> takes less than a second, deletion takes about 130 seconds:
> 
> # time -p ip -b nexthop.batch
> real 0.29
> user 0.01
> sys 0.15
> 
> # time -p ip link set dev dummy10 down
> real 131.03
> user 0.06
> sys 0.52

snip...

> Since nexthops are always deleted under RTNL, synchronize_net() can be
> used instead. It will call synchronize_rcu_expedited() which only blocks
> for several microseconds as opposed to multiple milliseconds like
> synchronize_rcu().
> 
> With this patch deletion of 16k nexthops takes less than a second:
> 
> # time -p ip link set dev dummy10 down
> real 0.12
> user 0.00
> sys 0.04

That's a nice result! Well done! I can't really speak to whether or not
there is any horrible side effect of using synchronize_rcu_expedited(),
but FWIW:

Reviewed-by: Jesse Brandeburg <[email-masked]>

> 
> Tested with fib_nexthops.sh which includes torture tests that prompted
> the initial change:
> 
> # ./fib_nexthops.sh
> ...
> Tests passed: 134
> Tests failed:   0
> 
> Fixes: 90f33bffa382 ("nexthops: don't modify published nexthop groups")
> Signed-off-by: Ido Schimmel <[email-masked]>

Nice fix, good commit message, thanks!

> ---
>  net/ipv4/nexthop.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
> index 8c0f17c6863c..0dc43ad28eb9 100644
> --- a/net/ipv4/nexthop.c
> +++ b/net/ipv4/nexthop.c
> @@ -845,7 +845,7 @@ static void remove_nexthop_from_groups(struct net *net, struct nexthop *nh,
>  		remove_nh_grp_entry(net, nhge, nlinfo);
>  
>  	/* make sure all see the newly published array before releasing rtnl */
> -	synchronize_rcu();
> +	synchronize_net();
>  }
>  
>  static void remove_nexthop_group(struct nexthop *nh, struct nl_info *nlinfo)