Subject: Re: [PATCH RESEND net-next] net: Do synchronize_rcu() in ip6mr_sk_done() only if this is needed

From: Eric Dumazet <[email-masked]>
Date: Tue, 06 Mar 2018 08:50:21 -0800

On Tue, 2018-03-06 at 19:24 +0300, Kirill Tkhai wrote:
> After unshare test kworker hangs for ages:
> 
>     $ while :; do unshare -n true; done &
> 
>     $ perf report <kworker>
>     -   88,82%     0,00%  kworker/u16:0  [kernel.vmlinux]  [k]
> cleanup_net
>          cleanup_net
>        - ops_exit_list.isra.9
>           - 85,68% igmp6_net_exit
>              - 53,31% sock_release
>                 - inet_release
>                    - 25,33% rawv6_close
>                       - ip6mr_sk_done
>                          + 23,38% synchronize_rcu
> 
> Keep in mind, this perf report shows the time a function was
> executing, and
> it does not show the time, it was sleeping. But it's easy to imagine,
> how
> much it is sleeping, if synchronize_rcu() execution takes the most
> time.
> Top shows the kworker R time is less then 1%.
> 
> This happen, because of in ip6mr_sk_done() we do too many
> synchronize_rcu(),
> even for the sockets, that are not referenced in mr_table, and which
> are not
> need it. So, the progress of kworker becomes very slow.
> 
> The patch introduces apparent solution, and it makes ip6mr_sk_done()
> to skip
> synchronize_rcu() for sockets, that are not need that. After the
> patch,
> kworker becomes able to warm the cpu up as expected.
> 
> Signed-off-by: Kirill Tkhai <[email-masked]>
> ---
>  net/ipv6/ip6mr.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
> index 2a38f9de45d3..290a8d0d5eac 100644
> --- a/net/ipv6/ip6mr.c
> +++ b/net/ipv6/ip6mr.c
> @@ -1485,7 +1485,9 @@ int ip6mr_sk_done(struct sock *sk)
>  		}
>  	}
>  	rtnl_unlock();
> -	synchronize_rcu();
> +
> +	if (!err)
> +		synchronize_rcu();
>  

But... what is this synchronize_rcu() doing exactly ?

This was added in 8571ab479a6e1ef46ead5ebee567e128a422767c

("ip6mr: Make mroute_sk rcu-based")

Typically on a delete, the synchronize_rcu() would be needed before
freeing the deleted object.

But nowadays we have better way : SOCK_RCU_FREE