[PATCH net-next v10 0/2] net: af_packet: optimize retire operation

[PATCH net-next v10 0/2] net: af_packet: optimize retire operation

[Xin Zhao]

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.

---
Changes in v10:
- kactive_blk_num (K) is incremented on block close. last_kactive_blk_num (L)
  is set to match K on block open and each timer. So the only time that they
  differ is if a block is closed in tpacket_rcv and no new block could be
  opened. So the origin check L==K in timer callback only skip the case 'no
  new block to open'. If we remove L==K check, it will make prb_curr_blk_in_use
  check earlier, which will not cause any side effect
  as suggested by Willem de Bruijn.
- Submit a precursor patch that removes last_kactive_blk_num
  as suggested by Willem de Bruijn.

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.
- Always update last_kactive_blk_num before hrtimer callback return as the origin
  logic does, as suggested by Willem de Bruijn.
  In tpacket_rcv, it may call prb_close_block but do not call prb_open_block in
  prb_dispatch_next_block, leading to inconsistency between last_kactive_blk_num
  and kactive_blk_num. In hrtimer callback, we should update last_kactive_blk_num
  in this case.
- Remove 'refresh_timer:' label which is not needed while I change goto logic to
  if-else implementation.
- Link to v9: https://lore.kernel.org/netdev/20250828155127.3076551-1-jackzxcui1989@163.com/

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.
- Link to v8: https://lore.kernel.org/all/20250827150131.2193485-1-jackzxcui1989@163.com/

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.
  The setting expire time is monotonic, so if we do not update the expire time to the
  retire_blk_timer when it is not in callback, it will not cause problem if we skip
  the timeout event and update it when find out that expire_ktime is bigger than the
  expire time of retire_blk_timer.
- Use hrtimer_set_expires instead of hrtimer_forward_now.
  The end time for retiring each block is not fixed because when network packets are
  received quickly, blocks are retired rapidly, and the new block retire time needs
  to be recalculated. However, hrtimer_forward_now increments the previous timeout
  by an interval, which is not correct.
- The expire time is monotonic, so if we do not update the expire time to the
  retire_blk_timer when it is not in callback, it will not cause problem if we skip
  the timeout event and update it when find out that expire_ktime is bigger than the
  expire time of retire_blk_timer.
- Adding the 'bool callback' parameter back is intended to more accurately determine
  whether we are inside the hrtimer callback when executing
  _prb_refresh_rx_retire_blk_timer. This ensures that we only update the hrtimer's
  timeout value within the hrtimer callback.
- Link to v7: https://lore.kernel.org/all/20250822132051.266787-1-jackzxcui1989@163.com/

Changes in v6:
- Use hrtimer_is_queued instead to check whether it is within the callback function.
  So do not need to add 'bool callback' parameter to _prb_refresh_rx_retire_blk_timer
  as suggested by Willem de Bruijn;
- Do not need local_irq_save and local_irq_restore to protect the race of the timer
  callback running in softirq context or the open_block from tpacket_rcv in process
  context
  as suggested by Willem de Bruijn;
- Link to v6: https://lore.kernel.org/all/20250820092925.2115372-1-jackzxcui1989@163.com/

Changes in v5:
- Remove the unnecessary comments at the top of the _prb_refresh_rx_retire_blk_timer,
  branch is self-explanatory enough
  as suggested by Willem de Bruijn;
- Indentation of _prb_refresh_rx_retire_blk_timer, align with first argument on
  previous line
  as suggested by Willem de Bruijn;
- Do not call hrtimer_start within the hrtimer callback
  as suggested by Willem de Bruijn
  So add 'bool callback' parameter to _prb_refresh_rx_retire_blk_timer to indicate
  whether it is within the callback function. Use hrtimer_forward_now instead of
  hrtimer_start when it is in the callback function and is doing prb_open_block.
- Link to v5: https://lore.kernel.org/all/20250819091447.1199980-1-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_forward_now to set the expiration
  time as it is more commonly used compared to hrtimer_set_expires.
  as suggested by Willem de Bruijn;
- Delete the comments to explain why hrtimer_set_expires(not hrtimer_forward_now)
  is used, as we do not use hrtimer_set_expires any more;
- Link to v4: https://lore.kernel.org/all/20250818050233.155344-1-jackzxcui1989@163.com/

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/

Xin Zhao (2):
  net: af_packet: remove last_kactive_blk_num field
  net: af_packet: Use hrtimer to do the retire operation

 net/packet/af_packet.c | 107 +++++++++++------------------------------
 net/packet/diag.c      |   2 +-
 net/packet/internal.h  |  12 +----
 3 files changed, 32 insertions(+), 89 deletions(-)

-- 
2.34.1

[PATCH net-next v10 1/2] net: af_packet: remove last_kactive_blk_num field

[Xin Zhao]

kactive_blk_num (K) is incremented on block close. last_kactive_blk_num (L)
is set to match K on block open and each timer. So the only time that they
differ is if a block is closed in tpacket_rcv and no new block could be
opened.
So the origin check L==K in timer callback only skip the case 'no new block
to open'. If we remove L==K check, it will make prb_curr_blk_in_use check
earlier, which will not cause any side effect.

Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
---
 net/packet/af_packet.c | 60 ++++++++++++++++++++----------------------
 net/packet/internal.h  |  6 -----
 2 files changed, 28 insertions(+), 38 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index a7017d7f0..d4eb4a4fe 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -669,7 +669,6 @@ static void init_prb_bdqc(struct packet_sock *po,
 	p1->knum_blocks	= req_u->req3.tp_block_nr;
 	p1->hdrlen = po->tp_hdrlen;
 	p1->version = po->tp_version;
-	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;
@@ -693,7 +692,6 @@ static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
 {
 	mod_timer(&pkc->retire_blk_timer,
 			jiffies + pkc->tov_in_jiffies);
-	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
 }
 
 /*
@@ -750,38 +748,36 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
 		write_unlock(&pkc->blk_fill_in_prog_lock);
 	}
 
-	if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
-		if (!frozen) {
-			if (!BLOCK_NUM_PKTS(pbd)) {
-				/* An empty block. Just refresh the timer. */
-				goto refresh_timer;
-			}
-			prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
-			if (!prb_dispatch_next_block(pkc, po))
-				goto refresh_timer;
-			else
-				goto out;
+	if (!frozen) {
+		if (!BLOCK_NUM_PKTS(pbd)) {
+			/* An empty block. Just refresh the timer. */
+			goto refresh_timer;
+		}
+		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 {
-			/* Case 1. Queue was frozen because user-space was
-			 *	   lagging behind.
+			/* 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
+			 * block and restart the timer.
+			 * opening a block thaws the queue,restarts timer
+			 * Thawing/timer-refresh is a side effect.
 			 */
-			if (prb_curr_blk_in_use(pbd)) {
-				/*
-				 * Ok, user-space is still behind.
-				 * So just refresh the timer.
-				 */
-				goto refresh_timer;
-			} else {
-			       /* 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
-				* block and restart the timer.
-				* opening a block thaws the queue,restarts timer
-				* Thawing/timer-refresh is a side effect.
-				*/
-				prb_open_block(pkc, pbd);
-				goto out;
-			}
+			prb_open_block(pkc, pbd);
+			goto out;
 		}
 	}
 
diff --git a/net/packet/internal.h b/net/packet/internal.h
index 1e743d031..d367b9f93 100644
--- a/net/packet/internal.h
+++ b/net/packet/internal.h
@@ -24,12 +24,6 @@ struct tpacket_kbdq_core {
 	unsigned short	kactive_blk_num;
 	unsigned short	blk_sizeof_priv;
 
-	/* last_kactive_blk_num:
-	 * trick to see if user-space has caught up
-	 * in order to avoid refreshing timer when every single pkt arrives.
-	 */
-	unsigned short	last_kactive_blk_num;
-
 	char		*pkblk_start;
 	char		*pkblk_end;
 	int		kblk_size;
-- 
2.34.1

[PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

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 v8:
- Simplify the logic related to setting timeout.

Changes in v7:
- Only update the hrtimer expire time within the hrtimer callback.

Changes in v1:
- Do not add another config for the current changes.

---
 net/packet/af_packet.c | 79 +++++++++---------------------------------
 net/packet/diag.c      |  2 +-
 net/packet/internal.h  |  6 ++--
 3 files changed, 20 insertions(+), 67 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index d4eb4a4fe..3e3bb4216 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,
@@ -671,29 +650,22 @@ 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.
@@ -717,7 +689,7 @@ 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)
 {
 	struct packet_sock *po =
 		timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
@@ -730,9 +702,6 @@ 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 +718,16 @@ 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 +736,12 @@ 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,
@@ -917,7 +873,6 @@ 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..f8cfd9213 100644
--- a/net/packet/internal.h
+++ b/net/packet/internal.h
@@ -20,7 +20,6 @@ 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;
 
@@ -39,12 +38,11 @@ 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

Re: [PATCH net-next v10 1/2] net: af_packet: remove last_kactive_blk_num field

[Willem de Bruijn]

Xin Zhao wrote:
> kactive_blk_num (K) is incremented on block close. last_kactive_blk_num (L)
> is set to match K on block open and each timer. So the only time that they
> differ is if a block is closed in tpacket_rcv and no new block could be
> opened.
> So the origin check L==K in timer callback only skip the case 'no new block
> to open'. If we remove L==K check, it will make prb_curr_blk_in_use check
> earlier, which will not cause any side effect.
> 
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Willem de Bruijn]

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>

Tiny style point that is probably not worth respinning for.

Otherwise

Reviewed-by: Willem de Bruijn <willemb@google.com>



> ---
> Changes in v8:
> - Simplify the logic related to setting timeout.
> 
> Changes in v7:
> - Only update the hrtimer expire time within the hrtimer callback.
> 
> Changes in v1:
> - Do not add another config for the current changes.
> 
> ---
>  net/packet/af_packet.c | 79 +++++++++---------------------------------
>  net/packet/diag.c      |  2 +-
>  net/packet/internal.h  |  6 ++--
>  3 files changed, 20 insertions(+), 67 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d4eb4a4fe..3e3bb4216 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,
> @@ -671,29 +650,22 @@ 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));

req_u is not aligned with the line above.

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Sun, 2025-08-31 at 21:21 -0400, Willem wrote:

> > -		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));
> 
> req_u is not aligned with the line above.

I have some questions regarding the alignment here. According to the alignment requirements,
req_u should be aligned below the po variable. However, if it is aligned below po, the line
will become very long, which may affect readability. In this special case, can I align it to
prb_calc_retire_blk_tmo instead, or should I continue to align it to the po variable?


What should I do next?
Should I change the alignment, and resend PATCH with the reviewed information of version 10?

Thanks
Xin Zhao

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Willem de Bruijn]

Xin Zhao wrote:
> On Sun, 2025-08-31 at 21:21 -0400, Willem wrote:
> 
> > > -		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));
> > 
> > req_u is not aligned with the line above.
> 
> I have some questions regarding the alignment here. According to the alignment requirements,
> req_u should be aligned below the po variable. However, if it is aligned below po, the line
> will become very long, which may affect readability. In this special case, can I align it to
> prb_calc_retire_blk_tmo instead, or should I continue to align it to the po variable?

The (minor) issue here is with the second req_u. Which is one space
off from the argument above. See checkpath.

In general, the line length and break rules are documented in the
kernel coding style page, which checkpatch follows.
 
> 
> What should I do next?
> Should I change the alignment, and resend PATCH with the reviewed information of version 10?

I did not think this one space was worth resending, so I added my
Reviewed-by. Others may disagree, but so far no other opinions.

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Mon, 2025-09-01 at 09:35 -0400, Willem wrote:

> > On Sun, 2025-08-31 at 21:21 -0400, Willem wrote:
> > 
> > > > -		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));
> > > 
> > > req_u is not aligned with the line above.
> > 
> > I have some questions regarding the alignment here. According to the alignment requirements,
> > req_u should be aligned below the po variable. However, if it is aligned below po, the line
> > will become very long, which may affect readability. In this special case, can I align it to
> > prb_calc_retire_blk_tmo instead, or should I continue to align it to the po variable?
> 
> The (minor) issue here is with the second req_u. Which is one space
> off from the argument above. See checkpath.
> 
> In general, the line length and break rules are documented in the
> kernel coding style page, which checkpatch follows.
> 
> > 
> > What should I do next?
> > Should I change the alignment, and resend PATCH with the reviewed information of version 10?
> 
> I did not think this one space was worth resending, so I added my
> Reviewed-by. Others may disagree, but so far no other opinions.

Okay, I will not resend the patch if there are no other opinions.


Thanks
Xin Zhao

Re: [PATCH net-next v10 1/2] net: af_packet: remove last_kactive_blk_num field

[Jason Xing]

On Sun, Aug 31, 2025 at 6:10 PM Xin Zhao <jackzxcui1989@163.com> wrote:
>
> kactive_blk_num (K) is incremented on block close. last_kactive_blk_num (L)
> is set to match K on block open and each timer. So the only time that they
> differ is if a block is closed in tpacket_rcv and no new block could be
> opened.
> So the origin check L==K in timer callback only skip the case 'no new block
> to open'. If we remove L==K check, it will make prb_curr_blk_in_use check
> earlier, which will not cause any side effect.

I believe the above commit message needs to be revised:
1) the above sentence (starting from 'if we remove L....') means
nothing because your modification doesn't change the behaviour when
the queue is not frozen.
2) lack of proofs/reasons on why exposing the prb_open_block() logic doesn't
cause side effects. It's the key proof that shows to future readers to
make sure this patch will not bring trouble.

>
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>

It was suggested by Willem, so please add:
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>

So far, it looks good to me as well:
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

And I will finish reviewing the other patch by tomorrow :)

Thanks,
Jason



> ---
>  net/packet/af_packet.c | 60 ++++++++++++++++++++----------------------
>  net/packet/internal.h  |  6 -----
>  2 files changed, 28 insertions(+), 38 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index a7017d7f0..d4eb4a4fe 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -669,7 +669,6 @@ static void init_prb_bdqc(struct packet_sock *po,
>         p1->knum_blocks = req_u->req3.tp_block_nr;
>         p1->hdrlen = po->tp_hdrlen;
>         p1->version = po->tp_version;
> -       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;
> @@ -693,7 +692,6 @@ static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>  {
>         mod_timer(&pkc->retire_blk_timer,
>                         jiffies + pkc->tov_in_jiffies);
> -       pkc->last_kactive_blk_num = pkc->kactive_blk_num;
>  }
>
>  /*
> @@ -750,38 +748,36 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>                 write_unlock(&pkc->blk_fill_in_prog_lock);
>         }
>
> -       if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
> -               if (!frozen) {
> -                       if (!BLOCK_NUM_PKTS(pbd)) {
> -                               /* An empty block. Just refresh the timer. */
> -                               goto refresh_timer;
> -                       }
> -                       prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
> -                       if (!prb_dispatch_next_block(pkc, po))
> -                               goto refresh_timer;
> -                       else
> -                               goto out;
> +       if (!frozen) {
> +               if (!BLOCK_NUM_PKTS(pbd)) {
> +                       /* An empty block. Just refresh the timer. */
> +                       goto refresh_timer;
> +               }
> +               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 {
> -                       /* Case 1. Queue was frozen because user-space was
> -                        *         lagging behind.
> +                       /* 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
> +                        * block and restart the timer.
> +                        * opening a block thaws the queue,restarts timer
> +                        * Thawing/timer-refresh is a side effect.
>                          */
> -                       if (prb_curr_blk_in_use(pbd)) {
> -                               /*
> -                                * Ok, user-space is still behind.
> -                                * So just refresh the timer.
> -                                */
> -                               goto refresh_timer;
> -                       } else {
> -                              /* 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
> -                               * block and restart the timer.
> -                               * opening a block thaws the queue,restarts timer
> -                               * Thawing/timer-refresh is a side effect.
> -                               */
> -                               prb_open_block(pkc, pbd);
> -                               goto out;
> -                       }
> +                       prb_open_block(pkc, pbd);
> +                       goto out;
>                 }
>         }
>
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 1e743d031..d367b9f93 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -24,12 +24,6 @@ struct tpacket_kbdq_core {
>         unsigned short  kactive_blk_num;
>         unsigned short  blk_sizeof_priv;
>
> -       /* last_kactive_blk_num:
> -        * trick to see if user-space has caught up
> -        * in order to avoid refreshing timer when every single pkt arrives.
> -        */
> -       unsigned short  last_kactive_blk_num;
> -
>         char            *pkblk_start;
>         char            *pkblk_end;
>         int             kblk_size;
> --
> 2.34.1
>
>

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Jason Xing]

On Sun, Aug 31, 2025 at 6:09 PM Xin Zhao <jackzxcui1989@163.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.
>
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
> ---
> Changes in v8:
> - Simplify the logic related to setting timeout.
>
> Changes in v7:
> - Only update the hrtimer expire time within the hrtimer callback.
>
> Changes in v1:
> - Do not add another config for the current changes.
>
> ---
>  net/packet/af_packet.c | 79 +++++++++---------------------------------
>  net/packet/diag.c      |  2 +-
>  net/packet/internal.h  |  6 ++--
>  3 files changed, 20 insertions(+), 67 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d4eb4a4fe..3e3bb4216 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,
> @@ -671,29 +650,22 @@ 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);

You expect to see it start at the setsockopt phase? Even if it's far
from the real use of recv at the moment.

>         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.
> @@ -717,7 +689,7 @@ 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)
>  {
>         struct packet_sock *po =
>                 timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
> @@ -730,9 +702,6 @@ 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 +718,16 @@ 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 +736,12 @@ 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,
> @@ -917,7 +873,6 @@ 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);

Could you say more on why you remove this here and only reset/update
the expiry time in the timer handler? Probably I missed something
appearing in the previous long discussion.

>
>         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..f8cfd9213 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -20,7 +20,6 @@ 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;
>
> @@ -39,12 +38,11 @@ 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;
>  };

The whole structure needs a new organization?

Before:
        /* size: 152, cachelines: 3, members: 22 */
        /* sum members: 144, holes: 2, sum holes: 8 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 24 bytes */
After:
        /* size: 176, cachelines: 3, members: 19 */
        /* sum members: 163, holes: 4, sum holes: 13 */
        /* paddings: 1, sum paddings: 4 */
        /* forced alignments: 1, forced holes: 1, sum forced holes: 6 */
        /* last cacheline: 48 bytes */

Thanks,
Jason

>
>  struct pgv {
> --
> 2.34.1
>
>

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Jason Xing]

On Tue, Sep 2, 2025 at 11:43 PM Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> On Sun, Aug 31, 2025 at 6:09 PM Xin Zhao <jackzxcui1989@163.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.
> >
> > Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
> > ---
> > Changes in v8:
> > - Simplify the logic related to setting timeout.
> >
> > Changes in v7:
> > - Only update the hrtimer expire time within the hrtimer callback.
> >
> > Changes in v1:
> > - Do not add another config for the current changes.
> >
> > ---
> >  net/packet/af_packet.c | 79 +++++++++---------------------------------
> >  net/packet/diag.c      |  2 +-
> >  net/packet/internal.h  |  6 ++--
> >  3 files changed, 20 insertions(+), 67 deletions(-)
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index d4eb4a4fe..3e3bb4216 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;

One more review from my side is that as to the removal of
delete_blk_timer, I'm afraid it deserves a clarification in the commit
message.

> > -       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,
> > @@ -671,29 +650,22 @@ 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);
>
> You expect to see it start at the setsockopt phase? Even if it's far
> from the real use of recv at the moment.
>
> >         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.
> > @@ -717,7 +689,7 @@ 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)
> >  {
> >         struct packet_sock *po =
> >                 timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
> > @@ -730,9 +702,6 @@ 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 +718,16 @@ 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 +736,12 @@ 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,
> > @@ -917,7 +873,6 @@ 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);
>
> Could you say more on why you remove this here and only reset/update
> the expiry time in the timer handler? Probably I missed something
> appearing in the previous long discussion.

I gradually understand your thought behind this modification. You're
trying to move the timer operation out of prb_open_block() and then
spread the timer operation into each caller.

You probably miss the following call trace:
packet_current_rx_frame() -> __packet_lookup_frame_in_block() ->
prb_open_block() -> _prb_refresh_rx_retire_blk_timer()
?

May I ask why bother introducing so many changes like this instead of
leaving it as-is?

Thanks,
Jason

>
> >
> >         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..f8cfd9213 100644
> > --- a/net/packet/internal.h
> > +++ b/net/packet/internal.h
> > @@ -20,7 +20,6 @@ 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;
> >
> > @@ -39,12 +38,11 @@ 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;
> >  };
>
> The whole structure needs a new organization?
>
> Before:
>         /* size: 152, cachelines: 3, members: 22 */
>         /* sum members: 144, holes: 2, sum holes: 8 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 24 bytes */
> After:
>         /* size: 176, cachelines: 3, members: 19 */
>         /* sum members: 163, holes: 4, sum holes: 13 */
>         /* paddings: 1, sum paddings: 4 */
>         /* forced alignments: 1, forced holes: 1, sum forced holes: 6 */
>         /* last cacheline: 48 bytes */
>
> Thanks,
> Jason
>
> >
> >  struct pgv {
> > --
> > 2.34.1
> >
> >

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Tue, Sep 2, 2025 at 23:43 +0800 Jason Xing <kerneljasonxing@gmail.com> wrote:

> >         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);
> 
> You expect to see it start at the setsockopt phase? Even if it's far
> from the real use of recv at the moment.
> 
> >         prb_open_block(p1, pbd);
> >  }

Before applying this patch, init_prb_bdqc also start the timer by mod_timer:

init_prb_bdqc
  prb_open_block
    _prb_refresh_rx_retire_blk_timer
      mod_timer

So the current timer's start time is almost the same as it was before applying
the patch.


> > @@ -917,7 +873,6 @@ 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);
> 
> Could you say more on why you remove this here and only reset/update
> the expiry time in the timer handler? Probably I missed something
> appearing in the previous long discussion.
> 
> >
> >         smp_wmb();
> >  }

In the description of [PATCH net-next v10 0/2] net: af_packet: optimize retire operation:

Changes in v7:
  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.

Neither hrtimer_set_expires nor hrtimer_forward_now is allowed when the hrtimer has
already been enqueued. Therefore, the only place where the hrtimer timeout can be set is
within the callback, at which point the hrtimer is in a non-enqueued state and can have
its timeout set.


Changes in v8:
  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.

This paragraph explains that if the block's retire timeout is not adjusted within
the timer callback, it will only result in an earlier-than-expected retire timeout,
which is not problematic. Therefore, it is unnecessary to increase the logical complexity
to ensure block retire timeout occurs as expected each time.


> The whole structure needs a new organization?
> 
> Before:
>         /* size: 152, cachelines: 3, members: 22 */
>         /* sum members: 144, holes: 2, sum holes: 8 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 24 bytes */
> After:
>         /* size: 176, cachelines: 3, members: 19 */
>         /* sum members: 163, holes: 4, sum holes: 13 */
>         /* paddings: 1, sum paddings: 4 */
>         /* forced alignments: 1, forced holes: 1, sum forced holes: 6 */
>         /* last cacheline: 48 bytes */

What about the following organization:?

/* kbdq - kernel block descriptor queue */
struct tpacket_kbdq_core {
	struct pgv	*pkbdq;
	unsigned int	feature_req_word;
	unsigned int	hdrlen;
	unsigned short	kactive_blk_num;
	unsigned short	blk_sizeof_priv;
	unsigned char	reset_pending_on_curr_blk;

	char		*pkblk_start;
	char		*pkblk_end;
	int		kblk_size;
	unsigned int	max_frame_len;
	unsigned int	knum_blocks;
	char		*prev;
	char		*nxt_offset;

	unsigned short  version;
	
	uint64_t	knxt_seq_num;
	struct sk_buff	*skb;

	rwlock_t	blk_fill_in_prog_lock;

	/* timer to retire an outstanding block */
	struct hrtimer  retire_blk_timer;

	/* Default is set to 8ms */
#define DEFAULT_PRB_RETIRE_TOV	(8)

	ktime_t		interval_ktime;
};



Thanks
Xin Zhao

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Wed, Sep 3, 2025 at 00:42 +0800 Jason Xing <kerneljasonxing@gmail.com> wrote:

> One more review from my side is that as to the removal of
> delete_blk_timer, I'm afraid it deserves a clarification in the commit
> message.
> 
> > > -       spin_unlock_bh(&rb_queue->lock);
> > > -
> > > -       prb_del_retire_blk_timer(pkc);
> > > -}
> > > -

In the description of [PATCH net-next v10 0/2] net: af_packet: optimize retire operation:

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;

I will also emphasize the removal of delete_blk_timer in the commit message for this 2/2
commit. The updated commit message for the 2/2 patch is as follows：

Changes in v8:
- Simplify the logic related to setting timeout.
- Delete delete_blk_timer field, hrtimer_cancel will check and wait until
  the timer callback return.


> I gradually understand your thought behind this modification. You're
> trying to move the timer operation out of prb_open_block() and then
> spread the timer operation into each caller.
> 
> You probably miss the following call trace:
> packet_current_rx_frame() -> __packet_lookup_frame_in_block() ->
> prb_open_block() -> _prb_refresh_rx_retire_blk_timer()
> ?
> 
> May I ask why bother introducing so many changes like this instead of
> leaving it as-is?




Consider the following timing sequence:
timer   cpu0 (softirq context, hrtimer timeout)                cpu1 (process context)
0       hrtimer_run_softirq
1         __hrtimer_run_queues
2           __run_hrtimer
3             prb_retire_rx_blk_timer_expired
4               spin_lock(&po->sk.sk_receive_queue.lock);
5               _prb_refresh_rx_retire_blk_timer
6                 hrtimer_forward_now
7               spin_unlock(&po->sk.sk_receive_queue.lock)
8             raw_spin_lock_irq(&cpu_base->lock);              tpacket_rcv
9             enqueue_hrtimer                                    spin_lock(&sk->sk_receive_queue.lock);
10                                                               packet_current_rx_frame
11                                                                 __packet_lookup_frame_in_block
12            finish enqueue_hrtimer                                 prb_open_block
13                                                                     _prb_refresh_rx_retire_blk_timer
14                                                                       hrtimer_is_queued(&pkc->retire_blk_timer) == true
15                                                                       hrtimer_forward_now
16                                                                         WARN_ON
On cpu0 in the timing sequence above, enqueue_hrtimer is not protected by sk_receive_queue.lock,
while the hrtimer_forward_now is not protected by raw_spin_lock_irq(&cpu_base->lock).

In my previous email, I provided an explanation. As a supplement, I would
like to reiterate a paragraph from my earlier response to Willem.
The point is that when the hrtimer is in the enqueued state, you cannot
call interfaces like hrtimer_forward_now. The kernel has a WARN_ON check
in hrtimer_forward_now for this reason. Similarly, you also cannot call
interfaces like hrtimer_set_expires. The kernel does not include a WARN_ON
check in hrtimer_set_expires to avoid increasing the code size, as
hrtimer_set_expires is an inline function.


Thanks
Xin Zhao

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Jason Xing]

On Thu, Sep 4, 2025 at 12:17 AM Xin Zhao <jackzxcui1989@163.com> wrote:
>
> On Tue, Sep 2, 2025 at 23:43 +0800 Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> > >         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);
> >
> > You expect to see it start at the setsockopt phase? Even if it's far
> > from the real use of recv at the moment.
> >
> > >         prb_open_block(p1, pbd);
> > >  }
>
> Before applying this patch, init_prb_bdqc also start the timer by mod_timer:
>
> init_prb_bdqc
>   prb_open_block
>     _prb_refresh_rx_retire_blk_timer
>       mod_timer
>
> So the current timer's start time is almost the same as it was before applying
> the patch.
>
>
> > > @@ -917,7 +873,6 @@ 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);
> >
> > Could you say more on why you remove this here and only reset/update
> > the expiry time in the timer handler? Probably I missed something
> > appearing in the previous long discussion.
> >
> > >
> > >         smp_wmb();
> > >  }
>
> In the description of [PATCH net-next v10 0/2] net: af_packet: optimize retire operation:
>
> Changes in v7:
>   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.
>
> Neither hrtimer_set_expires nor hrtimer_forward_now is allowed when the hrtimer has
> already been enqueued. Therefore, the only place where the hrtimer timeout can be set is
> within the callback, at which point the hrtimer is in a non-enqueued state and can have
> its timeout set.

Can we use hrtimer_is_queued() instead? Please see tcp_pacing_check()
as an example. But considering your following explanation, I think
it's okay now.

>
>
> Changes in v8:
>   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.
>
> This paragraph explains that if the block's retire timeout is not adjusted within
> the timer callback, it will only result in an earlier-than-expected retire timeout,
> which is not problematic. Therefore, it is unnecessary to increase the logical complexity
> to ensure block retire timeout occurs as expected each time.

Sounds fair.

>
>
> > The whole structure needs a new organization?
> >
> > Before:
> >         /* size: 152, cachelines: 3, members: 22 */
> >         /* sum members: 144, holes: 2, sum holes: 8 */
> >         /* paddings: 1, sum paddings: 4 */
> >         /* last cacheline: 24 bytes */
> > After:
> >         /* size: 176, cachelines: 3, members: 19 */
> >         /* sum members: 163, holes: 4, sum holes: 13 */
> >         /* paddings: 1, sum paddings: 4 */
> >         /* forced alignments: 1, forced holes: 1, sum forced holes: 6 */
> >         /* last cacheline: 48 bytes */
>
> What about the following organization:?
>
> /* kbdq - kernel block descriptor queue */
> struct tpacket_kbdq_core {
>         struct pgv      *pkbdq;
>         unsigned int    feature_req_word;
>         unsigned int    hdrlen;
>         unsigned short  kactive_blk_num;
>         unsigned short  blk_sizeof_priv;
>         unsigned char   reset_pending_on_curr_blk;
>
>         char            *pkblk_start;
>         char            *pkblk_end;
>         int             kblk_size;
>         unsigned int    max_frame_len;
>         unsigned int    knum_blocks;
>         char            *prev;
>         char            *nxt_offset;
>
>         unsigned short  version;
>
>         uint64_t        knxt_seq_num;
>         struct sk_buff  *skb;
>
>         rwlock_t        blk_fill_in_prog_lock;
>
>         /* timer to retire an outstanding block */
>         struct hrtimer  retire_blk_timer;
>
>         /* Default is set to 8ms */
> #define DEFAULT_PRB_RETIRE_TOV  (8)
>
>         ktime_t         interval_ktime;
> };

Could you share the result after running 'pahole --hex -C
tpacket_kbdq_core vmlinux'?

Thanks,
Jason

Re: [PATCH net-next v10 2/2] net: af_packet: Use hrtimer to do the retire operation

[Jason Xing]

On Thu, Sep 4, 2025 at 1:07 AM Xin Zhao <jackzxcui1989@163.com> wrote:
>
> On Wed, Sep 3, 2025 at 00:42 +0800 Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> > One more review from my side is that as to the removal of
> > delete_blk_timer, I'm afraid it deserves a clarification in the commit
> > message.
> >
> > > > -       spin_unlock_bh(&rb_queue->lock);
> > > > -
> > > > -       prb_del_retire_blk_timer(pkc);
> > > > -}
> > > > -
>
> In the description of [PATCH net-next v10 0/2] net: af_packet: optimize retire operation:
>
> 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;

I see the reason now :)

Please know that the history changes through versions will finally be
removed, only the official message that will be kept in the git. So
this kind of change, I think, should be clarified officially since
you're removing a structure member. Adding more descriptions will be
helpful to readers in the future. Thank you.

>
> I will also emphasize the removal of delete_blk_timer in the commit message for this 2/2
> commit. The updated commit message for the 2/2 patch is as follows：
>
> Changes in v8:
> - Simplify the logic related to setting timeout.
> - Delete delete_blk_timer field, hrtimer_cancel will check and wait until
>   the timer callback return.
>
>
> > I gradually understand your thought behind this modification. You're
> > trying to move the timer operation out of prb_open_block() and then
> > spread the timer operation into each caller.
> >
> > You probably miss the following call trace:
> > packet_current_rx_frame() -> __packet_lookup_frame_in_block() ->
> > prb_open_block() -> _prb_refresh_rx_retire_blk_timer()
> > ?
> >
> > May I ask why bother introducing so many changes like this instead of
> > leaving it as-is?
>
>
>
>
> Consider the following timing sequence:
> timer   cpu0 (softirq context, hrtimer timeout)                cpu1 (process context)
> 0       hrtimer_run_softirq
> 1         __hrtimer_run_queues
> 2           __run_hrtimer
> 3             prb_retire_rx_blk_timer_expired
> 4               spin_lock(&po->sk.sk_receive_queue.lock);
> 5               _prb_refresh_rx_retire_blk_timer
> 6                 hrtimer_forward_now
> 7               spin_unlock(&po->sk.sk_receive_queue.lock)
> 8             raw_spin_lock_irq(&cpu_base->lock);              tpacket_rcv
> 9             enqueue_hrtimer                                    spin_lock(&sk->sk_receive_queue.lock);
> 10                                                               packet_current_rx_frame
> 11                                                                 __packet_lookup_frame_in_block
> 12            finish enqueue_hrtimer                                 prb_open_block
> 13                                                                     _prb_refresh_rx_retire_blk_timer
> 14                                                                       hrtimer_is_queued(&pkc->retire_blk_timer) == true
> 15                                                                       hrtimer_forward_now
> 16                                                                         WARN_ON
> On cpu0 in the timing sequence above, enqueue_hrtimer is not protected by sk_receive_queue.lock,
> while the hrtimer_forward_now is not protected by raw_spin_lock_irq(&cpu_base->lock).
>
> In my previous email, I provided an explanation. As a supplement, I would
> like to reiterate a paragraph from my earlier response to Willem.
> The point is that when the hrtimer is in the enqueued state, you cannot

How about tring hrtimer_is_queued() beforehand?

IIUC, with this patch applied, we will lose the opportunity to refresh
the timer when the lookup function (in the above path I mentioned)
gets called compared to before. If the packet socket tries to look up
a new block and it doesn't update its expiry time, the timer will soon
wake up. Does it sound unreasonable?

Thanks,
Jason

> call interfaces like hrtimer_forward_now. The kernel has a WARN_ON check
> in hrtimer_forward_now for this reason. Similarly, you also cannot call
> interfaces like hrtimer_set_expires. The kernel does not include a WARN_ON
> check in hrtimer_set_expires to avoid increasing the code size, as
> hrtimer_set_expires is an inline function.
>
>
> Thanks
> Xin Zhao
>