[PATCH net-next v11 0/2] net: af_packet: optimize retire operation

[PATCH net-next v11 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 v11:
- Modify the commit message of PATCH 1/2 to explain the changes clearly
  as suggested by Jason Xing.
- 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 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.
- Link to v10: https://lore.kernel.org/all/20250831100822.1238795-1-jackzxcui1989@163.com/

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/all/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 | 136 +++++++++++++----------------------------
 net/packet/diag.c      |   2 +-
 net/packet/internal.h  |  14 ++---
 3 files changed, 49 insertions(+), 103 deletions(-)

-- 
2.34.1

[PATCH net-next v11 1/2] net: af_packet: remove last_kactive_blk_num field

[Xin Zhao]

kactive_blk_num (K) is only incremented on block close.
In timer callback prb_retire_rx_blk_timer_expired, except delete_blk_timer
is true, last_kactive_blk_num (L) is set to match kactive_blk_num (K) in
all cases. L is also set to match K in prb_open_block.
The only case K not equal to L is when scheduled by tpacket_rcv
and K is just incremented on block close but no new block could be opened,
so that it does not call prb_open_block in prb_dispatch_next_block.
This patch modifies the prb_retire_rx_blk_timer_expired function by simply 
removing the check for L == K. This patch just provides another checkpoint
to thaw the might-be-frozen block in any case. It doesn't have any effect
because __packet_lookup_frame_in_block() has the same logic and does it
again without this patch when detecting the ring is frozen. The patch only
advances checking the status of the ring.

Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/all/20250831100822.1238795-1-jackzxcui1989@163.com/
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 v11 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.

Reviewed-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/all/20250831100822.1238795-1-jackzxcui1989@163.com/
Signed-off-by: Xin Zhao <jackzxcui1989@163.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.
+ * 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.
  */
 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

Re: [PATCH net-next v11 2/2] net: af_packet: Use hrtimer to do the retire operation

[Jason Xing]

On Sun, Sep 7, 2025 at 1:30 AM 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.

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

>
> Reviewed-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
> Link: https://lore.kernel.org/all/20250831100822.1238795-1-jackzxcui1989@163.com/
> Signed-off-by: Xin Zhao <jackzxcui1989@163.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
>