Message-ID: <CAL+tcoA8pvTNSNKfxdGLm+chnMkUNkPCdJKMNacWxE3mAV4A6Q@mail.gmail.com>
Date: Sun, 7 Sep 2025 02:15:20 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Xin Zhao <jackzxcui1989@....com>
Cc: willemdebruijn.kernel@...il.com, edumazet@...gle.com, ferenc@...es.dev, 
	davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v11 2/2] net: af_packet: Use hrtimer to do the
 retire operation

On Sun, Sep 7, 2025 at 1:30 AM Xin Zhao <jackzxcui1989@....com> 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.

You might miss stating the reason why you move one structure member
delete_blk_timer.

>
> Reviewed-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
> Link: https://lore.kernel.org/all/20250831100822.1238795-1-jackzxcui1989@163.com/
> Signed-off-by: Xin Zhao <jackzxcui1989@....com>
> ---
> Changes in v11:
> - structure tpacket_kbdq_core needs a new organization
>   as suggested by Jason Xing.
> - Change the comments of prb_retire_rx_blk_timer_expired and prb_open_block
>   as suggested by Jason Xing.
>
> Changes in v9:
> - Remove the function prb_setup_retire_blk_timer and move hrtimer setup and start
>   logic into function init_prb_bdqc
>   as suggested by Willem de Bruijn.
> - Remove 'refresh_timer:' label which is not needed while I change goto logic to
>   if-else implementation.
>
> Changes in v8:
> - Delete delete_blk_timer field, as suggested by Willem de Bruijn,
>   hrtimer_cancel will check and wait until the timer callback return and ensure
>   enter enter callback again.
> - Simplify the logic related to setting timeout, as suggestd by Willem de Bruijn.
>   Currently timer callback just restarts itself unconditionally, so delete the
>   'out:' label, do not forward hrtimer in prb_open_block, call hrtimer_forward_now
>   directly and always return HRTIMER_RESTART.
>   The only special case is when prb_open_block is called from tpacket_rcv. That
>   would set the timeout further into the future than the already queued timer.
>   An earlier timeout is not problematic. No need to add complexity to avoid that.
>
> Changes in v7:
> - Only update the hrtimer expire time within the hrtimer callback.
>   When the callback return, without sk_buff_head lock protection, __run_hrtimer will
>   enqueue the timer if return HRTIMER_RESTART. Setting the hrtimer expires while
>   enqueuing a timer may cause chaos in the hrtimer red-black tree.
>
> Changes in v2:
> - Drop the tov_in_msecs field of tpacket_kbdq_core added by the patch
>   as suggested by Willem de Bruijn.
>
> 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' and performance details
>   in the changelog
>   as suggested by Eric Dumazet and Ferenc Fejes.
> - Delete the 'pkc->tov_in_msecs == 0' bounds check which is not necessary
>   as suggested by Willem de Bruijn.
>
> ---
>  net/packet/af_packet.c | 108 ++++++++++++-----------------------------
>  net/packet/diag.c      |   2 +-
>  net/packet/internal.h  |  10 ++--
>  3 files changed, 38 insertions(+), 82 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d4eb4a4fe..d55528776 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -203,8 +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 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_fill_rxhash(struct tpacket_kbdq_core *, struct tpacket3_hdr *);
>  static void prb_clear_rxhash(struct tpacket_kbdq_core *,
>                 struct tpacket3_hdr *);
> @@ -579,33 +578,13 @@ static __be16 vlan_get_protocol_dgram(const struct sk_buff *skb)
>         return proto;
>  }
>
> -static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
> -{
> -       timer_delete_sync(&pkc->retire_blk_timer);
> -}
> -
>  static void prb_shutdown_retire_blk_timer(struct packet_sock *po,
>                 struct sk_buff_head *rb_queue)
>  {
>         struct tpacket_kbdq_core *pkc;
>
>         pkc = GET_PBDQC_FROM_RB(&po->rx_ring);
> -
> -       spin_lock_bh(&rb_queue->lock);
> -       pkc->delete_blk_timer = 1;
> -       spin_unlock_bh(&rb_queue->lock);
> -
> -       prb_del_retire_blk_timer(pkc);
> -}
> -
> -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_cancel(&pkc->retire_blk_timer);
>  }
>
>  static int prb_calc_retire_blk_tmo(struct packet_sock *po,
> @@ -671,53 +650,40 @@ static void init_prb_bdqc(struct packet_sock *po,
>         p1->version = po->tp_version;
>         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);
>
>         p1->max_frame_len = p1->kblk_size - BLK_PLUS_PRIV(p1->blk_sizeof_priv);
>         prb_init_ft_ops(p1, req_u);
> -       prb_setup_retire_blk_timer(po);
> +       hrtimer_setup(&p1->retire_blk_timer, prb_retire_rx_blk_timer_expired,
> +                     CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
> +       hrtimer_start(&p1->retire_blk_timer, p1->interval_ktime,
> +                     HRTIMER_MODE_REL_SOFT);
>         prb_open_block(p1, pbd);
>  }
>
> -/*  Do NOT update the last_blk_num first.
> - *  Assumes sk_buff_head lock is held.
> - */
> -static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
> -{
> -       mod_timer(&pkc->retire_blk_timer,
> -                       jiffies + pkc->tov_in_jiffies);
> -}
> -
>  /*
> - * Timer logic:
> - * 1) We refresh the timer only when we open a block.
> - *    By doing this we don't waste cycles refreshing the timer
> - *       on packet-by-packet basis.
> - *
>   * With a 1MB block-size, on a 1Gbps line, it will take
>   * i) ~8 ms to fill a block + ii) memcpy etc.
>   * In this cut we are not accounting for the memcpy time.
>   *
> - * So, if the user sets the 'tmo' to 10ms then the timer
> - * will never fire while the block is still getting filled
> - * (which is what we want). However, the user could choose
> - * to close a block early and that's fine.
> - *
> - * But when the timer does fire, we check whether or not to refresh it.
>   * Since the tmo granularity is in msecs, it is not too expensive
>   * to refresh the timer, lets say every '8' msecs.
>   * Either the user can set the 'tmo' or we can derive it based on
>   * a) line-speed and b) block-size.
>   * prb_calc_retire_blk_tmo() calculates the tmo.
>   *
> + * The retire hrtimer expiration is unconditional and periodic.

The code itself is self-explanatory.

If I were you, I would move part of the comments like this into the
commit message to explicitly demonstrate the historical changes. But
it's only me.

> + * See comment in prb_open_block.
> + *
> + * If there are numerous packet sockets on the system, please set an
> + * appropriate timeout to avoid frequent enqueueing of hrtimers.
>   */
> -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)
>  {
>         struct packet_sock *po =
>                 timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
> @@ -730,9 +696,6 @@ 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))
> -               goto out;
> -
>         /* We only need to plug the race when the block is partially filled.
>          * tpacket_rcv:
>          *              lock(); increment BLOCK_NUM_PKTS; unlock()
> @@ -749,26 +712,16 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>         }
>
>         if (!frozen) {
> -               if (!BLOCK_NUM_PKTS(pbd)) {
> -                       /* An empty block. Just refresh the timer. */
> -                       goto refresh_timer;
> +               if (BLOCK_NUM_PKTS(pbd)) {
> +                       /* Not an empty block. Need retire the block. */
> +                       prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
> +                       prb_dispatch_next_block(pkc, po);
>                 }
> -               prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
> -               if (!prb_dispatch_next_block(pkc, po))
> -                       goto refresh_timer;
> -               else
> -                       goto out;
>         } else {
>                 /* Case 1. Queue was frozen because user-space was
>                  * lagging behind.
>                  */
> -               if (prb_curr_blk_in_use(pbd)) {
> -                       /*
> -                        * Ok, user-space is still behind.
> -                        * So just refresh the timer.
> -                        */
> -                       goto refresh_timer;
> -               } else {
> +               if (!prb_curr_blk_in_use(pbd)) {
>                         /* Case 2. queue was frozen,user-space caught up,
>                          * now the link went idle && the timer fired.
>                          * We don't have a block to close.So we open this
> @@ -777,15 +730,12 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>                          * Thawing/timer-refresh is a side effect.
>                          */
>                         prb_open_block(pkc, pbd);
> -                       goto out;
>                 }
>         }
>
> -refresh_timer:
> -       _prb_refresh_rx_retire_blk_timer(pkc);
> -
> -out:
> +       hrtimer_forward_now(&pkc->retire_blk_timer, pkc->interval_ktime);
>         spin_unlock(&po->sk.sk_receive_queue.lock);
> +       return HRTIMER_RESTART;
>  }
>
>  static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
> @@ -879,11 +829,18 @@ static void prb_thaw_queue(struct tpacket_kbdq_core *pkc)
>  }
>
>  /*
> - * Side effect of opening a block:
> + * prb_open_block is called by tpacket_rcv or timer callback.
>   *
> - * 1) prb_queue is thawed.
> - * 2) retire_blk_timer is refreshed.
> + * Reasons why NOT update hrtimer in prb_open_block:
> + * 1) It will increase complexity to distinguish the two caller scenario.
> + * 2) hrtimer_cancel and hrtimer_start need to be called if you want to update
> + * TMO of an already enqueued hrtimer, leading to complex shutdown logic.
>   *
> + * One side effect of NOT update hrtimer when called by tpacket_rcv is that
> + * a newly opened block triggered by tpacket_rcv may be retired earlier than
> + * expected. On the other hand, if timeout is updated in prb_open_block, the
> + * frequent reception of network packets that leads to prb_open_block being
> + * called may cause hrtimer to be removed and enqueued repeatedly.

Same here. The idea and reasoning behind this function/timer would be
better to move into the commit message as well.

Let Willem make the final call :)

Thanks,
Jason

>   */
>  static void prb_open_block(struct tpacket_kbdq_core *pkc1,
>         struct tpacket_block_desc *pbd1)
> @@ -917,7 +874,6 @@ 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);
>
>         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 d367b9f93..b76e645cd 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -20,10 +20,11 @@ struct tpacket_kbdq_core {
>         unsigned int    feature_req_word;
>         unsigned int    hdrlen;
>         unsigned char   reset_pending_on_curr_blk;
> -       unsigned char   delete_blk_timer;
>         unsigned short  kactive_blk_num;
>         unsigned short  blk_sizeof_priv;
>
> +       unsigned short  version;
> +
>         char            *pkblk_start;
>         char            *pkblk_end;
>         int             kblk_size;
> @@ -32,6 +33,7 @@ struct tpacket_kbdq_core {
>         uint64_t        knxt_seq_num;
>         char            *prev;
>         char            *nxt_offset;
> +
>         struct sk_buff  *skb;
>
>         rwlock_t        blk_fill_in_prog_lock;
> @@ -39,12 +41,10 @@ struct tpacket_kbdq_core {
>         /* Default is set to 8ms */
>  #define DEFAULT_PRB_RETIRE_TOV (8)
>
> -       unsigned short  retire_blk_tov;
> -       unsigned short  version;
> -       unsigned long   tov_in_jiffies;
> +       ktime_t         interval_ktime;
>
>         /* timer to retire an outstanding block */
> -       struct timer_list retire_blk_timer;
> +       struct hrtimer  retire_blk_timer;
>  };
>
>  struct pgv {
> --
> 2.34.1
>

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