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.

Delete delete_blk_timer field, because hrtimer_cancel will check and wait
until the timer callback return and ensure never enter callback again.

Simplify the logic related to setting timeout, only update the hrtimer
expire time within the hrtimer callback, no longer update the expire time
in prb_open_block which is called by tpacket_rcv or timer callback.
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.

The retire hrtimer expiration is unconditional and periodic. If there are
numerous packet sockets on the system, please set an appropriate timeout
to avoid frequent enqueueing of hrtimers.

Reviewed-by: Willem de Bruijn <[e-mail]>
Reviewed-by: Jason Xing <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Xin Zhao <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/packet/af_packet.c

···
 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 *);
···
 	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,
···
 	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.
- *
  */
-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);
···
 
 	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:
···
 	}
 
 	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
···
 			 * 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,
···
 }
 
 /*
- * 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)
···
 	pkc1->pkblk_end = pkc1->pkblk_start + pkc1->kblk_size;
 
 	prb_thaw_queue(pkc1);
-	_prb_refresh_rx_retire_blk_timer(pkc1);
 
 	smp_wmb();
 }

net/packet/diag.c

···
 	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 {

net/packet/internal.h

···
 	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;
···
 	uint64_t	knxt_seq_num;
 	char		*prev;
 	char		*nxt_offset;
+
 	struct sk_buff	*skb;
 
 	rwlock_t	blk_fill_in_prog_lock;
···
 	/* 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 {