Re: [PATCH net-next v3] net: af_packet: Use hrtimer to do the retire operation

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Xin Zhao wrote:
> In a system with high real-time requirements, the timeout mechanism of
> ordinary timers with jiffies granularity is insufficient to meet the
> demands for real-time performance. Meanwhile, the optimization of CPU
> usage with af_packet is quite significant. Use hrtimer instead of timer
> to help compensate for the shortcomings in real-time performance.
> In HZ=100 or HZ=250 system, the update of TP_STATUS_USER is not real-time
> enough, with fluctuations reaching over 8ms (on a system with HZ=250).
> This is unacceptable in some high real-time systems that require timely
> processing of network packets. By replacing it with hrtimer, if a timeout
> of 2ms is set, the update of TP_STATUS_USER can be stabilized to within
> 3 ms.
> 
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
> ---
> Changes in v3:
> - return HRTIMER_NORESTART when pkc->delete_blk_timer is true
>   as suggested by Willem de Bruijn;
> 
> Changes in v2:
> - Drop the tov_in_msecs field of tpacket_kbdq_core added by the patch
>   as suggested by Willem de Bruijn;
> - Link to v2: https://lore.kernel.org/all/20250815044141.1374446-1-jackzxcui1989@163.com/
> 
> Changes in v1:
> - Do not add another config for the current changes
>   as suggested by Eric Dumazet;
> - Mention the beneficial cases 'HZ=100 or HZ=250' in the changelog
>   as suggested by Eric Dumazet;
> - Add some performance details to the changelog
>   as suggested by Ferenc Fejes;
> - Delete the 'pkc->tov_in_msecs == 0' bounds check which is not necessary
>   as suggested by Willem de Bruijn;
> - Use hrtimer_set_expires instead of hrtimer_start_range_ns when retire timer needs update
>   as suggested by Willem de Bruijn. Start the hrtimer in prb_setup_retire_blk_timer;
> - Just return HRTIMER_RESTART directly as all cases return the same value
>   as suggested by Willem de Bruijn;
> - Link to v1: https://lore.kernel.org/all/20250813165201.1492779-1-jackzxcui1989@163.com/
> - Link to v0: https://lore.kernel.org/all/20250806055210.1530081-1-jackzxcui1989@163.com/
> 
> ---
> ---
>  net/packet/af_packet.c | 24 ++++++++++++++----------
>  net/packet/internal.h  |  3 +--
>  2 files changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index a7017d7f0..763b0c968 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -203,7 +203,7 @@ static void prb_retire_current_block(struct tpacket_kbdq_core *,
>  static int prb_queue_frozen(struct tpacket_kbdq_core *);
>  static void prb_open_block(struct tpacket_kbdq_core *,
>  		struct tpacket_block_desc *);
> -static void prb_retire_rx_blk_timer_expired(struct timer_list *);
> +static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *);
>  static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *);
>  static void prb_fill_rxhash(struct tpacket_kbdq_core *, struct tpacket3_hdr *);
>  static void prb_clear_rxhash(struct tpacket_kbdq_core *,
> @@ -581,7 +581,7 @@ static __be16 vlan_get_protocol_dgram(const struct sk_buff *skb)
>  
>  static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>  {
> -	timer_delete_sync(&pkc->retire_blk_timer);
> +	hrtimer_cancel(&pkc->retire_blk_timer);
>  }
>  
>  static void prb_shutdown_retire_blk_timer(struct packet_sock *po,
> @@ -603,9 +603,10 @@ static void prb_setup_retire_blk_timer(struct packet_sock *po)
>  	struct tpacket_kbdq_core *pkc;
>  
>  	pkc = GET_PBDQC_FROM_RB(&po->rx_ring);
> -	timer_setup(&pkc->retire_blk_timer, prb_retire_rx_blk_timer_expired,
> -		    0);
> -	pkc->retire_blk_timer.expires = jiffies;
> +	hrtimer_setup(&pkc->retire_blk_timer, prb_retire_rx_blk_timer_expired,
> +		      CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
> +	hrtimer_start(&pkc->retire_blk_timer, ms_to_ktime(pkc->retire_blk_tov),
> +		      HRTIMER_MODE_REL_SOFT);
>  }
>  
>  static int prb_calc_retire_blk_tmo(struct packet_sock *po,
> @@ -676,7 +677,6 @@ static void init_prb_bdqc(struct packet_sock *po,
>  	else
>  		p1->retire_blk_tov = prb_calc_retire_blk_tmo(po,
>  						req_u->req3.tp_block_size);
> -	p1->tov_in_jiffies = msecs_to_jiffies(p1->retire_blk_tov);
>  	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
>  	rwlock_init(&p1->blk_fill_in_prog_lock);
>  
> @@ -691,8 +691,8 @@ static void init_prb_bdqc(struct packet_sock *po,
>   */
>  static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>  {
> -	mod_timer(&pkc->retire_blk_timer,
> -			jiffies + pkc->tov_in_jiffies);
> +	hrtimer_set_expires(&pkc->retire_blk_timer,
> +			    ktime_add(ktime_get(), ms_to_ktime(pkc->retire_blk_tov)));

Here we cannot use hrtimer_add_expires for the same reason you gave in
the second version of the patch:

> Additionally, I think we cannot avoid using ktime_get, as the retire
> timeout for each block is not fixed. When there are a lot of network packets,
> a block can retire quickly, and if we do not re-fetch the time, the timeout
> duration may be set incorrectly.

Is that right?

Otherwise patch LGTM.

>  	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
>  }
>  
> @@ -719,8 +719,9 @@ static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>   * prb_calc_retire_blk_tmo() calculates the tmo.
>   *
>   */
> -static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
> +static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *t)
>  {
> +	enum hrtimer_restart ret = HRTIMER_RESTART;
>  	struct packet_sock *po =
>  		timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
>  	struct tpacket_kbdq_core *pkc = GET_PBDQC_FROM_RB(&po->rx_ring);
> @@ -732,8 +733,10 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>  	frozen = prb_queue_frozen(pkc);
>  	pbd = GET_CURR_PBLOCK_DESC_FROM_CORE(pkc);
>  
> -	if (unlikely(pkc->delete_blk_timer))
> +	if (unlikely(pkc->delete_blk_timer)) {
> +		ret = HRTIMER_NORESTART;
>  		goto out;
> +	}
>  
>  	/* We only need to plug the race when the block is partially filled.
>  	 * tpacket_rcv:
> @@ -790,6 +793,7 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>  
>  out:
>  	spin_unlock(&po->sk.sk_receive_queue.lock);
> +	return ret;
>  }
>  
>  static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 1e743d031..9812feb3d 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -47,10 +47,9 @@ struct tpacket_kbdq_core {
>  
>  	unsigned short  retire_blk_tov;
>  	unsigned short  version;
> -	unsigned long	tov_in_jiffies;
>  
>  	/* timer to retire an outstanding block */
> -	struct timer_list retire_blk_timer;
> +	struct hrtimer  retire_blk_timer;
>  };
>  
>  struct pgv {
> -- 
> 2.34.1
>