Message-ID: <willemdebruijn.kernel.5c18588531fd@gmail.com>
Date: Wed, 08 Oct 2025 09:38:29 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, 
 Kuniyuki Iwashima <kuniyu@...gle.com>, 
 Willem de Bruijn <willemb@...gle.com>, 
 netdev@...r.kernel.org, 
 eric.dumazet@...il.com, 
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH RFC net-next 4/4] net: allow busy connected flows to
 switch tx queues

Eric Dumazet wrote:
> This is a followup of commit 726e9e8b94b9 ("tcp: refine
> skb->ooo_okay setting") and to the prior commit in this series
> ("net: control skb->ooo_okay from skb_set_owner_w()")
> 
> skb->ooo_okay might never be set for bulk flows that always
> have at least one skb in a qdisc queue of NIC queue,
> especially if TX completion is delayed because of a stressed cpu.
> 
> The so-called "strange attractors" has caused many performance
> issues, we need to do better.
> 
> We have tried very hard to avoid reorders because TCP was
> not dealing with them nicely a decade ago.
> 
> Use the new net.core.txq_reselection_ms sysctl to let
> flows follow XPS and select a more efficient queue.
> 
> After this patch, we no longer have to make sure threads
> are pinned to cpus, they now can be migrated without
> adding too much spinlock/qdisc/TX completion pressure anymore.
> 
> TX completion part was problematic, because it added false sharing
> on various socket fields, but also added false sharing and spinlock
> contention in mm layers. Calling skb_orphan() from ndo_start_xmit()
> is not an option unfortunately.
> 
> Note for later: move sk->sk_tx_queue_mapping closer
> to sk_tx_queue_mapping_jiffies for better cache locality.
> 
> Tested:
> 
> Used a host with 32 TX queues, shared by groups of 8 cores.
> XPS setup :
> 
> echo ff >/sys/class/net/eth1/queue/tx-0/xps_cpus
> echo ff00 >/sys/class/net/eth1/queue/tx-1/xps_cpus
> echo ff0000 >/sys/class/net/eth1/queue/tx-2/xps_cpus
> echo ff000000 >/sys/class/net/eth1/queue/tx-3/xps_cpus
> echo ff,00000000 >/sys/class/net/eth1/queue/tx-4/xps_cpus
> echo ff00,00000000 >/sys/class/net/eth1/queue/tx-5/xps_cpus
> echo ff0000,00000000 >/sys/class/net/eth1/queue/tx-6/xps_cpus
> echo ff000000,00000000 >/sys/class/net/eth1/queue/tx-7/xps_cpus
> ...
> 
> Launched a tcp_stream with 15 threads and 1000 flows, initially affined to core 0-15
> 
> taskset -c 0-15 tcp_stream -T15 -F1000 -l1000 -c -H target_host
> 
> Checked that only queues 0 and 1 are used as instructed by XPS :
> tc -s qdisc show dev eth1|grep backlog|grep -v "backlog 0b 0p"
>  backlog 123489410b 1890p
>  backlog 69809026b 1064p
>  backlog 52401054b 805p

Just in case it's not clear to all readers, this implies MQ + some
per queue qdisc, right. Here MQ + FQ.
 
> Then force each thread to run on cpu 1,9,17,25,33,41,49,57,65,73,81,89,97,105,113,121
> 
> C=1;PID=`pidof tcp_stream`;for P in `ls /proc/$PID/task`; do taskset -pc $C $P; C=$(($C + 8));done
> 
> Set txq_reselection_ms to 1000
> echo 1000 > /proc/sys/net/core/txq_reselection_ms

Just curious: is the once per second heuristic based on anything. Is
it likely to set another (more aggressive) value here?

Series looks great to me, btw, thanks.

> 
> Check that the flows have migrated nicely:
> 
> tc -s qdisc show dev eth1|grep backlog|grep -v "backlog 0b 0p"
>  backlog 130508314b 1916p
>  backlog 8584380b 126p
>  backlog 8584380b 126p
>  backlog 8379990b 123p
>  backlog 8584380b 126p
>  backlog 8487484b 125p
>  backlog 8584380b 126p
>  backlog 8448120b 124p
>  backlog 8584380b 126p
>  backlog 8720640b 128p
>  backlog 8856900b 130p
>  backlog 8584380b 126p
>  backlog 8652510b 127p
>  backlog 8448120b 124p
>  backlog 8516250b 125p
>  backlog 7834950b 115p
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/net/sock.h | 40 +++++++++++++++++++---------------------
>  net/core/dev.c     | 27 +++++++++++++++++++++++++--
>  2 files changed, 44 insertions(+), 23 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 2794bc5c565424491a064049d3d76c3fb7ba1ed8..61f92bb03e00d7167cccfe70da16174f2b40f6de 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -485,6 +485,7 @@ struct sock {
>  	unsigned long		sk_pacing_rate; /* bytes per second */
>  	atomic_t		sk_zckey;
>  	atomic_t		sk_tskey;
> +	unsigned long		sk_tx_queue_mapping_jiffies;
>  	__cacheline_group_end(sock_write_tx);
>  
>  	__cacheline_group_begin(sock_read_tx);
> @@ -1984,6 +1985,14 @@ static inline int sk_receive_skb(struct sock *sk, struct sk_buff *skb,
>  	return __sk_receive_skb(sk, skb, nested, 1, true);
>  }
>  
> +/* This helper checks if a socket is a full socket,
> + * ie _not_ a timewait or request socket.
> + */
> +static inline bool sk_fullsock(const struct sock *sk)
> +{
> +	return (1 << sk->sk_state) & ~(TCPF_TIME_WAIT | TCPF_NEW_SYN_RECV);
> +}
> +
>  static inline void sk_tx_queue_set(struct sock *sk, int tx_queue)
>  {
>  	/* sk_tx_queue_mapping accept only upto a 16-bit value */
> @@ -1992,7 +2001,15 @@ static inline void sk_tx_queue_set(struct sock *sk, int tx_queue)
>  	/* Paired with READ_ONCE() in sk_tx_queue_get() and
>  	 * other WRITE_ONCE() because socket lock might be not held.
>  	 */
> -	WRITE_ONCE(sk->sk_tx_queue_mapping, tx_queue);
> +	if (READ_ONCE(sk->sk_tx_queue_mapping) != tx_queue) {
> +		WRITE_ONCE(sk->sk_tx_queue_mapping, tx_queue);
> +		WRITE_ONCE(sk->sk_tx_queue_mapping_jiffies, jiffies);
> +		return;
> +	}
> +
> +	/* Refresh sk_tx_queue_mapping_jiffies if too old. */
> +	if (time_is_before_jiffies(READ_ONCE(sk->sk_tx_queue_mapping_jiffies) + HZ))
> +		WRITE_ONCE(sk->sk_tx_queue_mapping_jiffies, jiffies);
>  }
>  
>  #define NO_QUEUE_MAPPING	USHRT_MAX
> @@ -2005,19 +2022,7 @@ static inline void sk_tx_queue_clear(struct sock *sk)
>  	WRITE_ONCE(sk->sk_tx_queue_mapping, NO_QUEUE_MAPPING);
>  }
>  
> -static inline int sk_tx_queue_get(const struct sock *sk)
> -{
> -	if (sk) {
> -		/* Paired with WRITE_ONCE() in sk_tx_queue_clear()
> -		 * and sk_tx_queue_set().
> -		 */
> -		int val = READ_ONCE(sk->sk_tx_queue_mapping);
> -
> -		if (val != NO_QUEUE_MAPPING)
> -			return val;
> -	}
> -	return -1;
> -}7
> +int sk_tx_queue_get(const struct sock *sk);
>  
>  static inline void __sk_rx_queue_set(struct sock *sk,
>  				     const struct sk_buff *skb,
> @@ -2945,13 +2950,6 @@ skb_sk_is_prefetched(struct sk_buff *skb)
>  #endif /* CONFIG_INET */
>  }
>  
> -/* This helper checks if a socket is a full socket,
> - * ie _not_ a timewait or request socket.
> - */
> -static inline bool sk_fullsock(const struct sock *sk)
> -{
> -	return (1 << sk->sk_state) & ~(TCPF_TIME_WAIT | TCPF_NEW_SYN_RECV);
> -}
>  
>  static inline bool
>  sk_is_refcounted(struct sock *sk)
> diff --git a/net/core/dev.c b/net/core/dev.c
> index a64cef2c537e98ee87776e6f8d3ca3d98f0711b3..c302fd5bb57894c6e5651b7adc8d033ac719070a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4591,6 +4591,30 @@ u16 dev_pick_tx_zero(struct net_device *dev, struct sk_buff *skb,
>  }
>  EXPORT_SYMBOL(dev_pick_tx_zero);
>  
> +int sk_tx_queue_get(const struct sock *sk)
> +{
> +	int val;
> +
> +	if (!sk)
> +		return -1;
> +	/* Paired with WRITE_ONCE() in sk_tx_queue_clear()
> +	 * and sk_tx_queue_set().
> +	 */
> +	val = READ_ONCE(sk->sk_tx_queue_mapping);
> +
> +	if (val == NO_QUEUE_MAPPING)
> +		return -1;
> +
> +	if (sk_fullsock(sk) &&
> +	    time_is_before_jiffies(
> +			READ_ONCE(sk->sk_tx_queue_mapping_jiffies) +
> +			READ_ONCE(sock_net(sk)->core.sysctl_txq_reselection)))
> +		return -1;
> +
> +	return val;
> +}
> +EXPORT_SYMBOL(sk_tx_queue_get);
> +
>  u16 netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
>  		     struct net_device *sb_dev)
>  {
> @@ -4606,8 +4630,7 @@ u16 netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
>  		if (new_index < 0)
>  			new_index = skb_tx_hash(dev, sb_dev, skb);
>  
> -		if (queue_index != new_index && sk &&
> -		    sk_fullsock(sk) &&
> +		if (sk && sk_fullsock(sk) &&
>  		    rcu_access_pointer(sk->sk_dst_cache))
>  			sk_tx_queue_set(sk, new_index);
>  
> -- 
> 2.51.0.710.ga91ca5db03-goog
> 

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