Re: [PATCH v2 net-next 4/5] tcp: Save unnecessary inet_twsk_purge() calls.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 29 Aug 2022 16:11:57 -0700
> On Mon, Aug 29, 2022 at 9:21 AM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > While destroying netns, we call inet_twsk_purge() in tcp_sk_exit_batch()
> > and tcpv6_net_exit_batch() for AF_INET and AF_INET6.  These commands
> > trigger the kernel to walk through the potentially big ehash twice even
> > though the netns has no TIME_WAIT sockets.
> >
> >   # ip netns add test
> >   # ip netns del test
> >
> > AF_INET6 uses module_init() to be loaded after AF_INET which uses
> > fs_initcall(), so tcpv6_net_ops is always registered after tcp_sk_ops.
> > Also, we clean up netns in the reverse order, so tcpv6_net_exit_batch()
> > is always called before tcp_sk_exit_batch().
> >
> > The characteristic enables us to save such unneeded iterations.  This
> > change eliminates the tax by the additional unshare() described in the
> > next patch to guarantee the per-netns ehash size.
> 
> Patch seems wrong to me, or not complete at least...
> 
> It seems you missed an existing check in inet_twsk_purge():
> 
> if ((tw->tw_family != family) ||
>     refcount_read(&twsk_net(tw)->ns.count))
>     continue;
> 
> To get rid of both IPv6 and IPv6 tw sockets, we currently need to call
> inet_twsk_purge() twice,
> with AF_INET and AF_INET6.

For the first call of AF_INET6, if tw_refcount is 1, the count is what
is set by tcp_sk_init() and there is no tw socket at least in the netns.

And same for the AF_INET, if tw_refcount is 0 and death_row is NULL,
there is no tw socket in the netns.

If all netns in net_exit_list satify the condition, we need not call
inet_twsk_purge().

However, one of netns in the list doesn't satisfy it, we have to call
inet_twsk_purge() twice.  Then we need to check the ns.count because
another netns in the list may be freed in tcp_sk_exit().

So, the optimisation works only when all netns in the batch list do not
have tw sockets.


> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> >  include/net/tcp.h        |  1 +
> >  net/ipv4/tcp_ipv4.c      |  6 ++++--
> >  net/ipv4/tcp_minisocks.c | 24 ++++++++++++++++++++++++
> >  net/ipv6/tcp_ipv6.c      |  2 +-
> >  4 files changed, 30 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index d10962b9f0d0..f60996c1d7b3 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -346,6 +346,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb);
> >  void tcp_rcv_space_adjust(struct sock *sk);
> >  int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp);
> >  void tcp_twsk_destructor(struct sock *sk);
> > +void tcp_twsk_purge(struct list_head *net_exit_list, int family);
> >  ssize_t tcp_splice_read(struct socket *sk, loff_t *ppos,
> >                         struct pipe_inode_info *pipe, size_t len,
> >                         unsigned int flags);
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index b07930643b11..f4a502d57d45 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -3109,8 +3109,10 @@ static void __net_exit tcp_sk_exit(struct net *net)
> >         if (net->ipv4.tcp_congestion_control)
> >                 bpf_module_put(net->ipv4.tcp_congestion_control,
> >                                net->ipv4.tcp_congestion_control->owner);
> > -       if (refcount_dec_and_test(&tcp_death_row->tw_refcount))
> > +       if (refcount_dec_and_test(&tcp_death_row->tw_refcount)) {
> >                 kfree(tcp_death_row);
> > +               net->ipv4.tcp_death_row = NULL;
> > +       }
> >  }
> >
> >  static int __net_init tcp_sk_init(struct net *net)
> > @@ -3210,7 +3212,7 @@ static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
> >  {
> >         struct net *net;
> >
> > -       inet_twsk_purge(&tcp_hashinfo, AF_INET);
> > +       tcp_twsk_purge(net_exit_list, AF_INET);
> >
> >         list_for_each_entry(net, net_exit_list, exit_list)
> >                 tcp_fastopen_ctx_destroy(net);
> > diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> > index 361aad67c6d6..9168c5a33344 100644
> > --- a/net/ipv4/tcp_minisocks.c
> > +++ b/net/ipv4/tcp_minisocks.c
> > @@ -347,6 +347,30 @@ void tcp_twsk_destructor(struct sock *sk)
> >  }
> >  EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
> >
> > +void tcp_twsk_purge(struct list_head *net_exit_list, int family)
> > +{
> > +       struct net *net;
> > +
> > +       list_for_each_entry(net, net_exit_list, exit_list) {
> > +               if (!net->ipv4.tcp_death_row)
> > +                       continue;
> > +
> > +               /* AF_INET6 using module_init() is always registered after
> > +                * AF_INET using fs_initcall() and cleaned up in the reverse
> > +                * order.
> > +                *
> > +                * The last refcount is decremented later in tcp_sk_exit().
> > +                */
> > +               if (IS_ENABLED(CONFIG_IPV6) && family == AF_INET6 &&
> > +                   refcount_read(&net->ipv4.tcp_death_row->tw_refcount) == 1)
> > +                       continue;
> > +
> > +               inet_twsk_purge(&tcp_hashinfo, family);
> > +               break;
> > +       }
> > +}
> > +EXPORT_SYMBOL_GPL(tcp_twsk_purge);
> > +
> >  /* Warning : This function is called without sk_listener being locked.
> >   * Be sure to read socket fields once, as their value could change under us.
> >   */
> > diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> > index 27b2fd98a2c4..9cbc7f0d7149 100644
> > --- a/net/ipv6/tcp_ipv6.c
> > +++ b/net/ipv6/tcp_ipv6.c
> > @@ -2229,7 +2229,7 @@ static void __net_exit tcpv6_net_exit(struct net *net)
> >
> >  static void __net_exit tcpv6_net_exit_batch(struct list_head *net_exit_list)
> >  {
> > -       inet_twsk_purge(&tcp_hashinfo, AF_INET6);
> > +       tcp_twsk_purge(net_exit_list, AF_INET6);
> >  }
> >
> >  static struct pernet_operations tcpv6_net_ops = {
> > --
> > 2.30.2
> >