Re: [PATCH v1 bpf-next 03/11] tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.

From: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
To: <kafai@fb.com>
Cc: <ast@kernel.org>, <benh@amazon.com>, <bpf@vger.kernel.org>,
	<daniel@iogearbox.net>, <davem@davemloft.net>,
	<edumazet@google.com>, <kuba@kernel.org>, <kuni1840@gmail.com>,
	<kuniyu@amazon.co.jp>, <linux-kernel@vger.kernel.org>,
	<netdev@vger.kernel.org>
Subject: Re: [PATCH v1 bpf-next 03/11] tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.
Date: Tue, 8 Dec 2020 15:27:14 +0900	[thread overview]
Message-ID: <20201208062714.96230-1-kuniyu@amazon.co.jp> (raw)
In-Reply-To: <20201207201438.kdlfdspusadadfvi@kafai-mbp.dhcp.thefacebook.com>

From:   Martin KaFai Lau <kafai@fb.com>
Date:   Mon, 7 Dec 2020 12:14:38 -0800
> On Sun, Dec 06, 2020 at 01:03:07AM +0900, Kuniyuki Iwashima wrote:
> > From:   Martin KaFai Lau <kafai@fb.com>
> > Date:   Fri, 4 Dec 2020 17:42:41 -0800
> > > On Tue, Dec 01, 2020 at 11:44:10PM +0900, Kuniyuki Iwashima wrote:
> > > [ ... ]
> > > > diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> > > > index fd133516ac0e..60d7c1f28809 100644
> > > > --- a/net/core/sock_reuseport.c
> > > > +++ b/net/core/sock_reuseport.c
> > > > @@ -216,9 +216,11 @@ int reuseport_add_sock(struct sock *sk, struct sock *sk2, bool bind_inany)
> > > >  }
> > > >  EXPORT_SYMBOL(reuseport_add_sock);
> > > >  
> > > > -void reuseport_detach_sock(struct sock *sk)
> > > > +struct sock *reuseport_detach_sock(struct sock *sk)
> > > >  {
> > > >  	struct sock_reuseport *reuse;
> > > > +	struct bpf_prog *prog;
> > > > +	struct sock *nsk = NULL;
> > > >  	int i;
> > > >  
> > > >  	spin_lock_bh(&reuseport_lock);
> > > > @@ -242,8 +244,12 @@ void reuseport_detach_sock(struct sock *sk)
> > > >  
> > > >  		reuse->num_socks--;
> > > >  		reuse->socks[i] = reuse->socks[reuse->num_socks];
> > > > +		prog = rcu_dereference(reuse->prog);
> > > Is it under rcu_read_lock() here?
> > 
> > reuseport_lock is locked in this function, and we do not modify the prog,
> > but is rcu_dereference_protected() preferable?
> > 
> > ---8<---
> > prog = rcu_dereference_protected(reuse->prog,
> > 				 lockdep_is_held(&reuseport_lock));
> > ---8<---
> It is not only reuse->prog.  Other things also require rcu_read_lock(),
> e.g. please take a look at __htab_map_lookup_elem().
> 
> The TCP_LISTEN sk (selected by bpf to be the target of the migration)
> is also protected by rcu.

Thank you, I will use rcu_read_lock() and rcu_dereference() in v3 patchset.

> I am surprised there is no WARNING in the test.
> Do you have the needed DEBUG_LOCK* config enabled?

Yes, DEBUG_LOCK* was 'y', but rcu_dereference() without rcu_read_lock()
does not show warnings...

> > > >  		if (sk->sk_protocol == IPPROTO_TCP) {
> > > > +			if (reuse->num_socks && !prog)
> > > > +				nsk = i == reuse->num_socks ? reuse->socks[i - 1] : reuse->socks[i];
> > > > +
> > > >  			reuse->num_closed_socks++;
> > > >  			reuse->socks[reuse->max_socks - reuse->num_closed_socks] = sk;
> > > >  		} else {
> > > > @@ -264,6 +270,8 @@ void reuseport_detach_sock(struct sock *sk)
> > > >  		call_rcu(&reuse->rcu, reuseport_free_rcu);
> > > >  out:
> > > >  	spin_unlock_bh(&reuseport_lock);
> > > > +
> > > > +	return nsk;
> > > >  }
> > > >  EXPORT_SYMBOL(reuseport_detach_sock);
> > > >  
> > > > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > > > index 1451aa9712b0..b27241ea96bd 100644
> > > > --- a/net/ipv4/inet_connection_sock.c
> > > > +++ b/net/ipv4/inet_connection_sock.c
> > > > @@ -992,6 +992,36 @@ struct sock *inet_csk_reqsk_queue_add(struct sock *sk,
> > > >  }
> > > >  EXPORT_SYMBOL(inet_csk_reqsk_queue_add);
> > > >  
> > > > +void inet_csk_reqsk_queue_migrate(struct sock *sk, struct sock *nsk)
> > > > +{
> > > > +	struct request_sock_queue *old_accept_queue, *new_accept_queue;
> > > > +
> > > > +	old_accept_queue = &inet_csk(sk)->icsk_accept_queue;
> > > > +	new_accept_queue = &inet_csk(nsk)->icsk_accept_queue;
> > > > +
> > > > +	spin_lock(&old_accept_queue->rskq_lock);
> > > > +	spin_lock(&new_accept_queue->rskq_lock);
> > > I am also not very thrilled on this double spin_lock.
> > > Can this be done in (or like) inet_csk_listen_stop() instead?
> > 
> > It will be possible to migrate sockets in inet_csk_listen_stop(), but I
> > think it is better to do it just after reuseport_detach_sock() becuase we
> > can select a different listener (almost) every time at a lower cost by
> > selecting the moved socket and pass it to inet_csk_reqsk_queue_migrate()
> > easily.
> I don't see the "lower cost" point.  Please elaborate.

In reuseport_select_sock(), we pass sk_hash of the request socket to
reciprocal_scale() and generate a random index for socks[] to select
a different listener every time.
On the other hand, we do not have request sockets in unhash path and
sk_hash of the listener is always 0, so we have to generate a random number
in another way. In reuseport_detach_sock(), we can use the index of the
moved socket, but we do not have it in inet_csk_listen_stop(), so we have
to generate a random number in inet_csk_listen_stop().
I think it is at lower cost to use the index of the moved socket.

> > sk_hash of the listener is 0, so we would have to generate a random number
> > in inet_csk_listen_stop().
> If I read it correctly, it is also passing 0 as the sk_hash to
> bpf_run_sk_reuseport() from reuseport_detach_sock().
> 
> Also, how is the sk_hash expected to be used?  I don't see
> it in the test.

I expected it should not be used in unhash path.
We do not have the request socket in unhash path and cannot pass a proper
sk_hash to bpf_run_sk_reuseport(). So, if u8 migration is
BPF_SK_REUSEPORT_MIGRATE_QUEUE, we cannot use sk_hash.