Re: [PATCH v2] net: af_packet: Use hrtimer to do the retire operation
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2025-08-15 10:12:48
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lkml
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 <redacted>
Please clearly label PATCH net-next and include a changelog and link to previous versions. See also other recently sent patches and https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html https://docs.kernel.org/process/submitting-patches.html
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--- net/packet/af_packet.c | 19 ++++++++++--------- net/packet/internal.h | 3 +-- 2 files changed, 11 insertions(+), 11 deletions(-)diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index bc438d0d9..c4746a9cb 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c@@ -203,7 +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 enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *); static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *); static void prb_fill_rxhash(struct tpacket_kbdq_core *, struct tpacket3_hdr *); static void prb_clear_rxhash(struct tpacket_kbdq_core *,@@ -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 +603,10 @@ 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); + hrtimer_start(&pkc->retire_blk_timer, ms_to_ktime(pkc->retire_blk_tov), + HRTIMER_MODE_REL_SOFT); } static int prb_calc_retire_blk_tmo(struct packet_sock *po,@@ -676,7 +677,6 @@ static void init_prb_bdqc(struct packet_sock *po, 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);
Since the hrtimer API takes ktime, and there is no other user for retire_blk_tov, remove that too and instead have interval_ktime.
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p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv; rwlock_init(&p1->blk_fill_in_prog_lock);@@ -691,8 +691,8 @@ static void init_prb_bdqc(struct packet_sock *po, */ static void _prb_refresh_rx_retire_blk_timer(struct tpacket_kbdq_core *pkc) { - mod_timer(&pkc->retire_blk_timer, - jiffies + pkc->tov_in_jiffies); + hrtimer_set_expires(&pkc->retire_blk_timer, + ktime_add(ktime_get(), ms_to_ktime(pkc->retire_blk_tov)));
More common for HRTIMER_RESTART timers is hrtimer_forward_now.
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pkc->last_kactive_blk_num = pkc->kactive_blk_num; }@@ -719,7 +719,7 @@ 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);@@ -790,6 +790,7 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t) out: spin_unlock(&po->sk.sk_receive_queue.lock); + return HRTIMER_RESTART;
This always restart the timer. But that is not the current behavior. Per prb_retire_rx_blk_timer_expired: * 1) We refresh the timer only when we open a block. Look at the five different paths that can reach label out. In particular, if the block is retired in this timer, and no new block is available to be opened, no timer should be armed.
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} static void prb_flush_block(struct tpacket_kbdq_core *pkc1,diff --git a/net/packet/internal.h b/net/packet/internal.h index 1e743d031..9812feb3d 100644 --- a/net/packet/internal.h +++ b/net/packet/internal.h@@ -47,10 +47,9 @@ struct tpacket_kbdq_core { unsigned short retire_blk_tov; 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