Message-Id: <20250818052441.172802-1-jackzxcui1989@163.com>
Date: Mon, 18 Aug 2025 13:24:41 +0800
From: Xin Zhao <jackzxcui1989@....com>
To: willemdebruijn.kernel@...il.com,
	edumazet@...gle.com,
	ferenc@...es.dev
Cc: davem@...emloft.net,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Xin Zhao <jackzxcui1989@....com>
Subject: [PATCH net-next v4] net: af_packet: Use hrtimer to do the retire operation

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@....com>

---
Changes in v4:
- Add 'bool start' to distinguish whether the call to _prb_refresh_rx_retire_blk_timer
  is for prb_open_block. When it is for prb_open_block, execute hrtimer_start to
  (re)start the hrtimer; otherwise, use hrtimer_set_expires to update the expiration
  time of the hrtimer
  as suggested by Willem de Bruijn;

Changes in v3:
- return HRTIMER_NORESTART when pkc->delete_blk_timer is true
  as suggested by Willem de Bruijn;
- Drop the retire_blk_tov field of tpacket_kbdq_core, add interval_ktime instead
  as suggested by Willem de Bruijn;
- Add comments to explain why hrtimer_set_expires(not hrtimer_forward_now) is used in
  _prb_refresh_rx_retire_blk_timer
  as suggested by Willem de Bruijn;
- Link to v3: https://lore.kernel.org/all/20250816170130.3969354-1-jackzxcui1989@163.com/

Changes in v2:
- Drop the tov_in_msecs field of tpacket_kbdq_core added by the patch
  as suggested by Willem de Bruijn;
- Link to v2: https://lore.kernel.org/all/20250815044141.1374446-1-jackzxcui1989@163.com/

Changes in v1:
- Do not add another config for the current changes
  as suggested by Eric Dumazet;
- Mention the beneficial cases 'HZ=100 or HZ=250' in the changelog
  as suggested by Eric Dumazet;
- Add some performance details to the changelog
  as suggested by Ferenc Fejes;
- Delete the 'pkc->tov_in_msecs == 0' bounds check which is not necessary
  as suggested by Willem de Bruijn;
- Use hrtimer_set_expires instead of hrtimer_start_range_ns when retire timer needs update
  as suggested by Willem de Bruijn. Start the hrtimer in prb_setup_retire_blk_timer;
- Just return HRTIMER_RESTART directly as all cases return the same value
  as suggested by Willem de Bruijn;
- Link to v1: https://lore.kernel.org/all/20250813165201.1492779-1-jackzxcui1989@163.com/
- Link to v0: https://lore.kernel.org/all/20250806055210.1530081-1-jackzxcui1989@163.com/
---
 net/packet/af_packet.c | 52 ++++++++++++++++++++++++++++--------------
 net/packet/diag.c      |  2 +-
 net/packet/internal.h  |  5 ++--
 3 files changed, 38 insertions(+), 21 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index a7017d7f0..5a1e80185 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -203,8 +203,8 @@ static void prb_retire_current_block(struct tpacket_kbdq_core *,
 static int prb_queue_frozen(struct tpacket_kbdq_core *);
 static void prb_open_block(struct tpacket_kbdq_core *,
 		struct tpacket_block_desc *);
-static void prb_retire_rx_blk_timer_expired(struct timer_list *);
-static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *);
+static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *);
+static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *, bool);
 static void prb_fill_rxhash(struct tpacket_kbdq_core *, struct tpacket3_hdr *);
 static void prb_clear_rxhash(struct tpacket_kbdq_core *,
 		struct tpacket3_hdr *);
@@ -581,7 +581,7 @@ static __be16 vlan_get_protocol_dgram(const struct sk_buff *skb)
 
 static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
 {
-	timer_delete_sync(&pkc->retire_blk_timer);
+	hrtimer_cancel(&pkc->retire_blk_timer);
 }
 
 static void prb_shutdown_retire_blk_timer(struct packet_sock *po,
@@ -603,9 +603,8 @@ static void prb_setup_retire_blk_timer(struct packet_sock *po)
 	struct tpacket_kbdq_core *pkc;
 
 	pkc = GET_PBDQC_FROM_RB(&po->rx_ring);
-	timer_setup(&pkc->retire_blk_timer, prb_retire_rx_blk_timer_expired,
-		    0);
-	pkc->retire_blk_timer.expires = jiffies;
+	hrtimer_setup(&pkc->retire_blk_timer, prb_retire_rx_blk_timer_expired,
+		      CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
 }
 
 static int prb_calc_retire_blk_tmo(struct packet_sock *po,
@@ -672,11 +671,10 @@ static void init_prb_bdqc(struct packet_sock *po,
 	p1->last_kactive_blk_num = 0;
 	po->stats.stats3.tp_freeze_q_cnt = 0;
 	if (req_u->req3.tp_retire_blk_tov)
-		p1->retire_blk_tov = req_u->req3.tp_retire_blk_tov;
+		p1->interval_ktime = ms_to_ktime(req_u->req3.tp_retire_blk_tov);
 	else
-		p1->retire_blk_tov = prb_calc_retire_blk_tmo(po,
-						req_u->req3.tp_block_size);
-	p1->tov_in_jiffies = msecs_to_jiffies(p1->retire_blk_tov);
+		p1->interval_ktime = ms_to_ktime(prb_calc_retire_blk_tmo(po,
+						req_u->req3.tp_block_size));
 	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
 	rwlock_init(&p1->blk_fill_in_prog_lock);
 
@@ -688,11 +686,27 @@ static void init_prb_bdqc(struct packet_sock *po,
 
 /*  Do NOT update the last_blk_num first.
  *  Assumes sk_buff_head lock is held.
+ *  We only need to (re)start an hrtimer in prb_open_block.
+ *  Otherwise, we just need to update the expiration time of the hrtimer.
  */
-static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
+static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc,
+		bool start)
 {
-	mod_timer(&pkc->retire_blk_timer,
-			jiffies + pkc->tov_in_jiffies);
+	if (start)
+		hrtimer_start(&pkc->retire_blk_timer, pkc->interval_ktime,
+			      HRTIMER_MODE_REL_SOFT);
+	else
+		/* We cannot use hrtimer_forward_now here because the function
+		 * _prb_refresh_rx_retire_blk_timer can be called not only when
+		 * the retire timer expires, but also when the kernel logic for
+		 * receiving network packets detects that a network packet has
+		 * filled up a block and calls prb_open_block to use the next
+		 * block. This can lead to a WARN_ON being triggered in
+		 * hrtimer_forward_now when it checks if the timer has already
+		 * been enqueued.
+		 */
+		hrtimer_set_expires(&pkc->retire_blk_timer,
+				    ktime_add(ktime_get(), pkc->interval_ktime));
 	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
 }
 
@@ -719,8 +733,9 @@ static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc)
  * prb_calc_retire_blk_tmo() calculates the tmo.
  *
  */
-static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
+static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *t)
 {
+	enum hrtimer_restart ret = HRTIMER_RESTART;
 	struct packet_sock *po =
 		timer_container_of(po, t, rx_ring.prb_bdqc.retire_blk_timer);
 	struct tpacket_kbdq_core *pkc = GET_PBDQC_FROM_RB(&po->rx_ring);
@@ -732,8 +747,10 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
 	frozen = prb_queue_frozen(pkc);
 	pbd = GET_CURR_PBLOCK_DESC_FROM_CORE(pkc);
 
-	if (unlikely(pkc->delete_blk_timer))
+	if (unlikely(pkc->delete_blk_timer)) {
+		ret = HRTIMER_NORESTART;
 		goto out;
+	}
 
 	/* We only need to plug the race when the block is partially filled.
 	 * tpacket_rcv:
@@ -786,10 +803,11 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
 	}
 
 refresh_timer:
-	_prb_refresh_rx_retire_blk_timer(pkc);
+	_prb_refresh_rx_retire_blk_timer(pkc, false);
 
 out:
 	spin_unlock(&po->sk.sk_receive_queue.lock);
+	return ret;
 }
 
 static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
@@ -921,7 +939,7 @@ static void prb_open_block(struct tpacket_kbdq_core *pkc1,
 	pkc1->pkblk_end = pkc1->pkblk_start + pkc1->kblk_size;
 
 	prb_thaw_queue(pkc1);
-	_prb_refresh_rx_retire_blk_timer(pkc1);
+	_prb_refresh_rx_retire_blk_timer(pkc1, true);
 
 	smp_wmb();
 }
diff --git a/net/packet/diag.c b/net/packet/diag.c
index 6ce1dcc28..c8f43e0c1 100644
--- a/net/packet/diag.c
+++ b/net/packet/diag.c
@@ -83,7 +83,7 @@ static int pdiag_put_ring(struct packet_ring_buffer *ring, int ver, int nl_type,
 	pdr.pdr_frame_nr = ring->frame_max + 1;
 
 	if (ver > TPACKET_V2) {
-		pdr.pdr_retire_tmo = ring->prb_bdqc.retire_blk_tov;
+		pdr.pdr_retire_tmo = ktime_to_ms(ring->prb_bdqc.interval_ktime);
 		pdr.pdr_sizeof_priv = ring->prb_bdqc.blk_sizeof_priv;
 		pdr.pdr_features = ring->prb_bdqc.feature_req_word;
 	} else {
diff --git a/net/packet/internal.h b/net/packet/internal.h
index 1e743d031..19d4f0b73 100644
--- a/net/packet/internal.h
+++ b/net/packet/internal.h
@@ -45,12 +45,11 @@ struct tpacket_kbdq_core {
 	/* Default is set to 8ms */
 #define DEFAULT_PRB_RETIRE_TOV	(8)
 
-	unsigned short  retire_blk_tov;
+	ktime_t		interval_ktime;
 	unsigned short  version;
-	unsigned long	tov_in_jiffies;
 
 	/* timer to retire an outstanding block */
-	struct timer_list retire_blk_timer;
+	struct hrtimer  retire_blk_timer;
 };
 
 struct pgv {
-- 
2.34.1

Powered by blists - more mailing lists