netdev - Re: [PATCH net-next] net: add a fast path in __netif

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAAVpQUD7LsivFAkuRAFsXGOUygQMJ_+PFQcYv1weKwwQtdv9Nw@mail.gmail.com>
Date: Sun, 19 Oct 2025 12:42:08 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, 
	Jiri Pirko <jiri@...nulli.us>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next] net: add a fast path in __netif_schedule()

On Fri, Oct 17, 2025 at 7:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Cpus serving NIC interrupts and specifically TX completions are often
> trapped in also restarting a busy qdisc (because qdisc was stopped by BQL
> or the driver's own flow control).
>
> When they call netdev_tx_completed_queue() or netif_tx_wake_queue(),
> they call __netif_schedule() so that the queue can be run
> later from net_tx_action() (involving NET_TX_SOFTIRQ)
>
> Quite often, by the time the cpu reaches net_tx_action(), another cpu
> grabbed the qdisc spinlock from __dev_xmit_skb(), and we spend too much
> time spinning on this lock.
>
> We can detect in __netif_schedule() if a cpu is already at a specific
> point in __dev_xmit_skb() where we have the guarantee the queue will
> be run.
>
> This patch gives a 13 % increase of throughput on an IDPF NIC (200Gbit),
> 32 TX qeues, sending UDP packets of 120 bytes.
>
> This also helps __qdisc_run() to not force a NET_TX_SOFTIRQ
> if another thread is waiting in __dev_xmit_skb()
>
> Before:
>
> sar -n DEV 5 5|grep eth1|grep Average
> Average:         eth1   1496.44 52191462.56    210.00 13369396.90      0.00      0.00      0.00     54.76
>
> After:
>
> sar -n DEV 5 5|grep eth1|grep Average
> Average:         eth1   1457.88 59363099.96    205.08 15206384.35      0.00      0.00      0.00     62.29
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Thanks!