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 <ktk...@virtuozzo.com>
> ---
> 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
