Re: [PATCH net-next v4] 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 v4:
> - Add 'bool start' to distinguish whether the call to _prb_refresh_rx_retire_blk_timer
>   is for prb_open_block. When it is for prb_open_block, execute hrtimer_start to
>   (re)start the hrtimer; otherwise, use hrtimer_set_expires to update the expiration
>   time of the hrtimer
>   as suggested by Willem de Bruijn;
> 
> Changes in v3:
> - return HRTIMER_NORESTART when pkc->delete_blk_timer is true
>   as suggested by Willem de Bruijn;
> - Drop the retire_blk_tov field of tpacket_kbdq_core, add interval_ktime instead
>   as suggested by Willem de Bruijn;
> - Add comments to explain why hrtimer_set_expires(not hrtimer_forward_now) is used in
>   _prb_refresh_rx_retire_blk_timer
>   as suggested by Willem de Bruijn;
> - Link to v3: https://lore.kernel.org/all/20250816170130.3969354-1-jackzxcui1989@163.com/
> 
> 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 | 52 ++++++++++++++++++++++++++++--------------
>  net/packet/diag.c      |  2 +-
>  net/packet/internal.h  |  5 ++--
>  3 files changed, 38 insertions(+), 21 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index a7017d7f0..5a1e80185 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -203,8 +203,8 @@ 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 void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *);
> +static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *);
> +static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *, bool);
>  static void prb_fill_rxhash(struct tpacket_kbdq_core *, struct tpacket3_hdr *);
>  static void prb_clear_rxhash(struct tpacket_kbdq_core *,
>  		struct tpacket3_hdr *);
> @@ -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 +603,8 @@ 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);
>  }
>  
>  static int prb_calc_retire_blk_tmo(struct packet_sock *po,
> @@ -672,11 +671,10 @@ static void init_prb_bdqc(struct packet_sock *po,
>  	p1->last_kactive_blk_num = 0;
>  	po->stats.stats3.tp_freeze_q_cnt = 0;
>  	if (req_u->req3.tp_retire_blk_tov)
> -		p1->retire_blk_tov = req_u->req3.tp_retire_blk_tov;
> +		p1->interval_ktime = ms_to_ktime(req_u->req3.tp_retire_blk_tov);
>  	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->interval_ktime = ms_to_ktime(prb_calc_retire_blk_tmo(po,
> +						req_u->req3.tp_block_size));
>  	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
>  	rwlock_init(&p1->blk_fill_in_prog_lock);
>  
> @@ -688,11 +686,27 @@ static void init_prb_bdqc(struct packet_sock *po,
>  
>  /*  Do NOT update the last_blk_num first.
>   *  Assumes sk_buff_head lock is held.
> + *  We only need to (re)start an hrtimer in prb_open_block.
> + *  Otherwise, we just need to update the expiration time of the hrtimer.

"We" don't do anything in the middle of a computation. Anyway, branch is
self explanatory enough, can drop comment.

>   */
> -static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
> +static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc,
> +		bool start)

Indentation, align with first argument on previous line

>  {
> -	mod_timer(&pkc->retire_blk_timer,
> -			jiffies + pkc->tov_in_jiffies);
> +	if (start)
> +		hrtimer_start(&pkc->retire_blk_timer, pkc->interval_ktime,
> +			      HRTIMER_MODE_REL_SOFT);

It's okay to call this from inside a timer callback itself and return
HRTIMER_RESTART? I don't know off the top of my head.

> +	else
> +		/* We cannot use hrtimer_forward_now here because the function
> +		 * _prb_refresh_rx_retire_blk_timer can be called not only when
> +		 * the retire timer expires, but also when the kernel logic for
> +		 * receiving network packets detects that a network packet has
> +		 * filled up a block and calls prb_open_block to use the next
> +		 * block. This can lead to a WARN_ON being triggered in
> +		 * hrtimer_forward_now when it checks if the timer has already
> +		 * been enqueued.
> +		 */

As discussed, this will be changed in v5.

> +		hrtimer_set_expires(&pkc->retire_blk_timer,
> +				    ktime_add(ktime_get(), pkc->interval_ktime));
>  	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
>  }
>  
> @@ -719,8 +733,9 @@ 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 +747,10 @@ 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:
> @@ -786,10 +803,11 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>  	}
>  
>  refresh_timer:
> -	_prb_refresh_rx_retire_blk_timer(pkc);
> +	_prb_refresh_rx_retire_blk_timer(pkc, false);
>  
>  out:
>  	spin_unlock(&po->sk.sk_receive_queue.lock);
> +	return ret;
>  }
>  
>  static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
> @@ -921,7 +939,7 @@ static void prb_open_block(struct tpacket_kbdq_core *pkc1,
>  	pkc1->pkblk_end = pkc1->pkblk_start + pkc1->kblk_size;
>  
>  	prb_thaw_queue(pkc1);
> -	_prb_refresh_rx_retire_blk_timer(pkc1);
> +	_prb_refresh_rx_retire_blk_timer(pkc1, true);
>  
>  	smp_wmb();
>  }
> diff --git a/net/packet/diag.c b/net/packet/diag.c
> index 6ce1dcc28..c8f43e0c1 100644
> --- a/net/packet/diag.c
> +++ b/net/packet/diag.c
> @@ -83,7 +83,7 @@ static int pdiag_put_ring(struct packet_ring_buffer *ring, int ver, int nl_type,
>  	pdr.pdr_frame_nr = ring->frame_max + 1;
>  
>  	if (ver > TPACKET_V2) {
> -		pdr.pdr_retire_tmo = ring->prb_bdqc.retire_blk_tov;
> +		pdr.pdr_retire_tmo = ktime_to_ms(ring->prb_bdqc.interval_ktime);
>  		pdr.pdr_sizeof_priv = ring->prb_bdqc.blk_sizeof_priv;
>  		pdr.pdr_features = ring->prb_bdqc.feature_req_word;
>  	} else {
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 1e743d031..19d4f0b73 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -45,12 +45,11 @@ struct tpacket_kbdq_core {
>  	/* Default is set to 8ms */
>  #define DEFAULT_PRB_RETIRE_TOV	(8)
>  
> -	unsigned short  retire_blk_tov;
> +	ktime_t		interval_ktime;
>  	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
>