[PATCH] net: af_packet: Use hrtimer to do the retire operation

[PATCH] 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.

Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
---
 net/packet/af_packet.c | 22 +++++++++++++---------
 net/packet/internal.h  |  4 ++--
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index bc438d0d9..3b3327544 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);
+	if (pkc->tov_in_msecs == 0)
+		pkc->tov_in_msecs = jiffies_to_msecs(1);
 }
 
 static int prb_calc_retire_blk_tmo(struct packet_sock *po,
@@ -676,7 +677,7 @@ 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);
+	p1->tov_in_msecs = p1->retire_blk_tov;
 	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
 	rwlock_init(&p1->blk_fill_in_prog_lock);
 
@@ -691,8 +692,8 @@ 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_start_range_ns(&pkc->retire_blk_timer,
+			       ms_to_ktime(pkc->tov_in_msecs), 0, HRTIMER_MODE_REL_SOFT);
 	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
 }
 
@@ -719,8 +720,9 @@ 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);
@@ -787,9 +789,11 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
 
 refresh_timer:
 	_prb_refresh_rx_retire_blk_timer(pkc);
+	ret = HRTIMER_RESTART;
 
 out:
 	spin_unlock(&po->sk.sk_receive_queue.lock);
+	return ret;
 }
 
 static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
diff --git a/net/packet/internal.h b/net/packet/internal.h
index 1e743d031..41df245e3 100644
--- a/net/packet/internal.h
+++ b/net/packet/internal.h
@@ -47,10 +47,10 @@ struct tpacket_kbdq_core {
 
 	unsigned short  retire_blk_tov;
 	unsigned short  version;
-	unsigned long	tov_in_jiffies;
+	unsigned long	tov_in_msecs;
 
 	/* 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: af_packet: Use hrtimer to do the retire operation

[Willem de Bruijn]

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 <jackzxcui1989@163.com>

This is a resubmit of the patch you yesterday [1]? While the
discussion on the original patch was ongoing too.

Net-next is also closed. See also see also
Documentation/process/maintainer-netdev.rst for the process.

I'll take a closer look later. Agreed in principle that it's
preferable to replace timer with hrtimer than to add a CONFIG to
select between them.

[1] https://lore.kernel.org/netdev/20250808032623.11485-1-jackzxcui1989@163.com/

Re: [PATCH] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Sat, 2025-08-09 at 21:45 +0800, Willem wrote:

> This is a resubmit of the patch you yesterday [1]? While the
> discussion on the original patch was ongoing too.

> Net-next is also closed. See also see also
> Documentation/process/maintainer-netdev.rst for the process.

> I'll take a closer look later. Agreed in principle that it's
> preferable to replace timer with hrtimer than to add a CONFIG to
> select between them.

> [1] https://lore.kernel.org/netdev/20250808032623.11485-1-jackzxcui1989@163.com/


Dear Willem，

Yes, you're right. I noticed that the title of the patch I submitted yesterday was
incorrect; it should be "use" instead of "user." Additionally, the title was a bit
too long, so I resubmitted it.
As you mentioned, I took Eric's advice and removed the new config, directly
modifying the code instead. Since the title has changed, I didn't use the "PATCH v1"
format.


Thanks
Xin Zhao

Re: [PATCH] net: af_packet: Use hrtimer to do the retire operation

[Willem de Bruijn]

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 <jackzxcui1989@163.com>
> ---
>  net/packet/af_packet.c | 22 +++++++++++++---------
>  net/packet/internal.h  |  4 ++--
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index bc438d0d9..3b3327544 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);
> +	if (pkc->tov_in_msecs == 0)
> +		pkc->tov_in_msecs = jiffies_to_msecs(1);

why is this bounds check needed now, while it was not needed when
converting to jiffies?

init_prb_bdqc will compute a retire_blk_tov if it is passed as zero,
by calling prb_calc_retire_blk_tmo.

>  }
>  
>  static int prb_calc_retire_blk_tmo(struct packet_sock *po,
> @@ -676,7 +677,7 @@ 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);
> +	p1->tov_in_msecs = p1->retire_blk_tov;
>  	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
>  	rwlock_init(&p1->blk_fill_in_prog_lock);
>  
> @@ -691,8 +692,8 @@ 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_start_range_ns(&pkc->retire_blk_timer,
> +			       ms_to_ktime(pkc->tov_in_msecs), 0, HRTIMER_MODE_REL_SOFT);

Just hrtimer_start if leaving the slack (delta_ns) as 0.

More importantly, this scheduled the timer, while the caller also
returns HRTIMER_RESTART. Should this just call hrtimer_set_expires or
hrtimer_forward.


>  	pkc->last_kactive_blk_num = pkc->kactive_blk_num;
>  }
>  
> @@ -719,8 +720,9 @@ 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);
> @@ -787,9 +789,11 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>  
>  refresh_timer:
>  	_prb_refresh_rx_retire_blk_timer(pkc);
> +	ret = HRTIMER_RESTART;

reinitializing a variable that was already set to the same value?
>  
>  out:
>  	spin_unlock(&po->sk.sk_receive_queue.lock);
> +	return ret;

just return HRTIMER_RESTART directly.
>  }
>  
>  static void prb_flush_block(struct tpacket_kbdq_core *pkc1,
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 1e743d031..41df245e3 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -47,10 +47,10 @@ struct tpacket_kbdq_core {
>  
>  	unsigned short  retire_blk_tov;
>  	unsigned short  version;
> -	unsigned long	tov_in_jiffies;
> +	unsigned long	tov_in_msecs;
>  
>  	/* 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: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Mon, 2025-08-11 at 22:48 +0800, Willem wrote:

> > @@ -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);
> > +	if (pkc->tov_in_msecs == 0)
> > +		pkc->tov_in_msecs = jiffies_to_msecs(1);

> why is this bounds check needed now, while it was not needed when
> converting to jiffies?
> 
> init_prb_bdqc will compute a retire_blk_tov if it is passed as zero,
> by calling prb_calc_retire_blk_tmo.

Dear Willem,

I am very grateful for your suggestion. I will delete this bounds check in the v1
PATCH.


> >  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_start_range_ns(&pkc->retire_blk_timer,
> > +			       ms_to_ktime(pkc->tov_in_msecs), 0, HRTIMER_MODE_REL_SOFT);

> Just hrtimer_start if leaving the slack (delta_ns) as 0.
> 
> More importantly, this scheduled the timer, while the caller also
> returns HRTIMER_RESTART. Should this just call hrtimer_set_expires or
> hrtimer_forward.

I will use hrtimer_set_expires here while using hrtimer_start in 
prb_setup_retire_blk_timer to start the timer.
Previously I used hrtimer_forward here, then encountered 
WARN_ON(timer->state & HRTIMER_STATE_ENQUEUED warning in the hrtimer_forward
function. This warning occurred because the tpacket_rcv function eventually calls
prb_open_block, which might call _prb_refresh_rx_retire_blk_timer at the same time
prb_retire_rx_blk_timer_expired might also call _prb_refresh_rx_retire_blk_timer,
leading to the warning.


> >  refresh_timer:
> >  	_prb_refresh_rx_retire_blk_timer(pkc);
> > +	ret = HRTIMER_RESTART;
> 
> reinitializing a variable that was already set to the same value?
> > 
> >  out:
> >  	spin_unlock(&po->sk.sk_receive_queue.lock);
> > +	return ret;
> 
> just return HRTIMER_RESTART directly.
> >  }

I will modify the patch based on these suggestions and upload it later.


Thanks
Xin Zhao

Re: [PATCH] net: af_packet: Use hrtimer to do the retire operation

[Xin Zhao]

On Fri, 2025-08-15 at 1:29 +0800, Willem wrote:

> > -	p1->tov_in_jiffies = msecs_to_jiffies(p1->retire_blk_tov);
> > +	p1->tov_in_msecs = p1->retire_blk_tov;
> 
> Can probably drop this field as it's the same as retire_blk_tov.
> 
> Or convert to ktime_t next_timer, to be able to replace ktime_get()
> with ktime_add_ms when rearming when calling hrtimer_set_expires.

Dear Willem,

Thank you for your suggestion.
I will drop the tov_in_msecs field in PATCH v2.
Additionally, I think we cannot avoid using ktime_get, as the retire
timeout for each block is not fixed. When there are a lot of network packets,
a block can retire quickly, and if we do not re-fetch the time, the timeout
duration may be set incorrectly.

Thanks
Xin Zhao