netdev - Re: [PATCH net-next v2 13/18] net/sched: act_mirred: Move the recursion counter struct netdev

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Message-ID: <75e10631-00a3-405a-b4d8-96b422ffbe41@redhat.com>
Date: Thu, 17 Apr 2025 10:29:05 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 netdev@...r.kernel.org, linux-rt-devel@...ts.linux.dev
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 v2 13/18] net/sched: act_mirred: Move the
 recursion counter struct netdev_xmit

On 4/14/25 6:07 PM, 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 3bbbc1a9860a3..4793ec42b1faa 100644
> --- a/include/linux/netdevice_xmit.h
> +++ b/include/linux/netdevice_xmit.h
> @@ -11,6 +11,9 @@ struct netdev_xmit {
>  #if IS_ENABLED(CONFIG_NF_DUP_NETDEV)
>  	u8 nf_dup_skb_recursion;
>  #endif
> +#if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
> +	u8 sched_mirred_nest;
> +#endif
>  };

How many of such recursion counters do you foresee will be needed?

AFAICS this one does not fit the existing hole anymore; the binary
layout before this series is:

 struct netdev_xmit {
                /* typedef u16 -> __u16 */ short unsigned int recursion;
                /*  2442     2 */
                /* typedef u8 -> __u8 */ unsigned char      more;
                /*  2444     1 */
                /* typedef u8 -> __u8 */ unsigned char
skip_txqueue;                /*  2445     1 */
        } net_xmit; /*  2442     4 */

        /* XXX 2 bytes hole, try to pack */

and this series already added 2 u8 fields. Since all the recursion
counters could be represented with less than 8 bits, perhaps using a
bitfield here could be worthy?!?

In any case I think we need explicit ack from the sched people.

> diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> index 5b38143659249..5f01f567c934d 100644
> --- a/net/sched/act_mirred.c
> +++ b/net/sched/act_mirred.c
> @@ -30,7 +30,29 @@ static LIST_HEAD(mirred_list);
>  static DEFINE_SPINLOCK(mirred_list_lock);
>  
>  #define MIRRED_NEST_LIMIT    4
> -static DEFINE_PER_CPU(unsigned int, mirred_nest_level);
> +
> +#ifndef CONFIG_PREEMPT_RT
> +static u8 tcf_mirred_nest_level_inc_return(void)
> +{
> +	return __this_cpu_inc_return(softnet_data.xmit.sched_mirred_nest);
> +}
> +
> +static void tcf_mirred_nest_level_dec(void)
> +{
> +	__this_cpu_dec(softnet_data.xmit.sched_mirred_nest);
> +}
> +
> +#else
> +static u8 tcf_mirred_nest_level_inc_return(void)
> +{
> +	return current->net_xmit.sched_mirred_nest++;
> +}
> +
> +static void tcf_mirred_nest_level_dec(void)
> +{
> +	current->net_xmit.sched_mirred_nest--;
> +}
> +#endif

There are already a few of this construct. Perhaps it would be worthy to
implement a netdev_xmit() helper returning a ptr to the whole struct and
use it to reduce the number of #ifdef

Thanks,

Paolo