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Date: Mon, 7 Jun 2021 18:17:45 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Dust Li <dust.li@...ux.alibaba.com>
Cc: David Miller <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Tony Lu <tonylu@...ux.alibaba.com>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next] tcp: avoid spurious loopback retransmit
On Mon, Jun 7, 2021 at 5:45 PM Dust Li <dust.li@...ux.alibaba.com> wrote:
>
> We found there are pretty much loopback TCP retransmitions
> on our online servers. Most of them are TLP retransmition.
>
> This is because for loopback communication, TLP is usally
> triggered about 2ms after the last packet was sent if no
> ACK was received within that period.
> This condition can be met if there are some kernel tasks
> running on that CPU for more than 2ms, which delays the
> softirq to process the sd->backlog.
>
> We sampled the loopback TLP retransmit on our online servers,
> and found an average 2K+ retransmit per hour. But in some cases,
> this can be much bigger, I found a peak 40 retrans/s on the
> same server.
> Actually, those loopback retransmitions are not a big problem as
> long as they don't happen too frequently. It's just spurious and
> meanless and waste some CPU cycles.
So, why do you send such a patch, adding a lot of code ?
>
> I also write a test module which just busy-loop in the kernel
> for more then 2ms, and the lo retransmition can be triggered
> every time we run the busy-loop on the target CPU.
> With this patch, the retransmition is gone and the throughput
> is not affected.
This makes no sense to me.
Are you running a pristine linux kernel, or some modified version of it ?
Why loopback is magical, compared to veth pair ?
The only case where skb_fclone_busy() might have an issue is when a driver
is very slow to perform tx completion (compared to possible RTX)
But given that the loopback driver does an skb_orphan() in its
ndo_start_xmit (loopback_xmit()),
the skb_fclone_busy() should not be fired at all.
(skb->sk is NULL, before packet is 'looped back')
It seems your diagnosis is wrong.
>
> Signed-off-by: Dust Li <dust.li@...ux.alibaba.com>
> ---
> include/linux/skbuff.h | 7 +++++--
> net/ipv4/tcp_output.c | 31 +++++++++++++++++++++++++++----
> net/xfrm/xfrm_policy.c | 2 +-
> 3 files changed, 33 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index dbf820a50a39..290e0a6a3a47 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1131,9 +1131,12 @@ struct sk_buff_fclones {
> * Returns true if skb is a fast clone, and its clone is not freed.
> * Some drivers call skb_orphan() in their ndo_start_xmit(),
> * so we also check that this didnt happen.
> + * For loopback, the skb maybe in the target sock's receive_queue
> + * we need to ignore that case.
> */
> static inline bool skb_fclone_busy(const struct sock *sk,
> - const struct sk_buff *skb)
> + const struct sk_buff *skb,
> + bool is_loopback)
> {
> const struct sk_buff_fclones *fclones;
>
> @@ -1141,7 +1144,7 @@ static inline bool skb_fclone_busy(const struct sock *sk,
>
> return skb->fclone == SKB_FCLONE_ORIG &&
> refcount_read(&fclones->fclone_ref) > 1 &&
> - READ_ONCE(fclones->skb2.sk) == sk;
> + is_loopback ? true : READ_ONCE(fclones->skb2.sk) == sk;
> }
>
> /**
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index bde781f46b41..f51a6a565678 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2771,6 +2771,20 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
> return true;
> }
>
> +static int sock_is_loopback(const struct sock *sk)
> +{
> + struct dst_entry *dst;
> + int loopback = 0;
> +
> + rcu_read_lock();
> + dst = rcu_dereference(sk->sk_dst_cache);
> + if (dst && dst->dev &&
> + (dst->dev->features & NETIF_F_LOOPBACK))
> + loopback = 1;
> + rcu_read_unlock();
> + return loopback;
> +}
> +
> /* Thanks to skb fast clones, we can detect if a prior transmit of
> * a packet is still in a qdisc or driver queue.
> * In this case, there is very little point doing a retransmit !
> @@ -2778,15 +2792,24 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
> static bool skb_still_in_host_queue(struct sock *sk,
> const struct sk_buff *skb)
> {
> - if (unlikely(skb_fclone_busy(sk, skb))) {
> - set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
> - smp_mb__after_atomic();
> - if (skb_fclone_busy(sk, skb)) {
> + bool is_loopback = sock_is_loopback(sk);
> +
> + if (unlikely(skb_fclone_busy(sk, skb, is_loopback))) {
> + if (!is_loopback) {
> + set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
> + smp_mb__after_atomic();
> + if (skb_fclone_busy(sk, skb, is_loopback)) {
> + NET_INC_STATS(sock_net(sk),
> + LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
> + return true;
> + }
> + } else {
> NET_INC_STATS(sock_net(sk),
> LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
> return true;
> }
> }
> +
> return false;
> }
>
> diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> index ce500f847b99..f8ea62a840e9 100644
> --- a/net/xfrm/xfrm_policy.c
> +++ b/net/xfrm/xfrm_policy.c
> @@ -2846,7 +2846,7 @@ static int xdst_queue_output(struct net *net, struct sock *sk, struct sk_buff *s
> struct xfrm_policy *pol = xdst->pols[0];
> struct xfrm_policy_queue *pq = &pol->polq;
>
> - if (unlikely(skb_fclone_busy(sk, skb))) {
> + if (unlikely(skb_fclone_busy(sk, skb, false))) {
> kfree_skb(skb);
> return 0;
> }
> --
> 2.19.1.3.ge56e4f7
>
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