Re: [PATCH RFC net-next 4/4] net: allow busy connected flows to switch tx queues

[Simon Horman <horms@kernel.org>]

On Wed, Oct 08, 2025 at 10:46:11AM +0000, 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
> 
> 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
> 
> 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@google.com>

Very nice :)

> ---
>  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;

nit: please add sk_tx_queue_mapping_jiffies to Kernel doc

>  	__cacheline_group_end(sock_write_tx);
>  
>  	__cacheline_group_begin(sock_read_tx);