Re: [PATCH net-next v10 2/2] 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 <redacted>
---
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;
 };