netdev - Re: [PATCH net-next v4 10/15] net/sched: act_mirred: Move the recursion counter struct netdev

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Message-ID: <235b93a5-6989-4131-9099-c0c03bb6afc1@redhat.com>
Date: Thu, 15 May 2025 11:55:23 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 netdev@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net-next v4 10/15] net/sched: act_mirred: Move the
 recursion counter struct netdev_xmit

CC sched maintainers.

On 5/12/25 11:27 AM, Sebastian Andrzej Siewior wrote:
> mirred_nest_level is a per-CPU variable and relies on disabled BH for its
> locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
> this data structure requires explicit locking.
> 
> Move mirred_nest_level to struct netdev_xmit as u8, provide wrappers.
> 
> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> Cc: Cong Wang <xiyou.wangcong@...il.com>
> Cc: Jiri Pirko <jiri@...nulli.us>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  include/linux/netdevice_xmit.h |  3 +++
>  net/sched/act_mirred.c         | 28 +++++++++++++++++++++++++---
>  2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/netdevice_xmit.h b/include/linux/netdevice_xmit.h
> index 38325e0702968..848735b3a7c02 100644
> --- a/include/linux/netdevice_xmit.h
> +++ b/include/linux/netdevice_xmit.h
> @@ -8,6 +8,9 @@ struct netdev_xmit {
>  #ifdef CONFIG_NET_EGRESS
>  	u8  skip_txqueue;
>  #endif
> +#if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
> +	u8 sched_mirred_nest;
> +#endif
>  };

The above struct is embedded into task_struct in RT build. The new field
 *should* use an existing hole. According to my weak knowledge in that
area the task_struct binary layout is a critical, an explicit ack from
SMEs would be nice.

Thanks,

Paolo