Subject: Re: [PATCH] net: Add trace events for all receive exit points

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 8 Nov 2018 15:25:44 -0500 (EST)

----- On Nov 8, 2018, at 2:56 PM, Geneviève Bastien [email-masked] wrote:

> Trace events are already present for the receive entry points, to indicate
> how the reception entered the stack.
> 
> This patch adds the corresponding exit trace events that will bound the
> reception such that all events occurring between the entry and the exit
> can be considered as part of the reception context. This greatly helps
> for dependency and root cause analyses.
> 
> Without this, it is impossible to determine whether a sched_wakeup
> event following a netif_receive_skb event is the result of the packet
> reception or a simple coincidence after further processing by the
> thread.

As a clarification: it is indeed not possible with tracepoint instrumentation,
which I think is your point here. It might be possible to do so by using other
mechanisms like kretprobes, but considering that the "entry" point was deemed
important enough to have a tracepoint, it would be good to add the matching exit
events.

Being able to link the packet reception entry/exit with wakeups is key to
allow tools to perform automated network stack latency analysis, so I think
the use case justifies adding the missing "exit" events.

It might be great if you can provide a glimpse of the wakeup dependency chain
analysis prototypes you have created, which rely on this new event, in order
to justify adding it.

Thanks,

Mathieu

> 
> Signed-off-by: Geneviève Bastien <[email-masked]>
> CC: Mathieu Desnoyers <[email-masked]>
> CC: Steven Rostedt <[email-masked]>
> CC: Ingo Molnar <[email-masked]>
> CC: David S. Miller <[email-masked]>
> ---
> include/trace/events/net.h | 59 ++++++++++++++++++++++++++++++++++++++
> net/core/dev.c             | 30 ++++++++++++++++---
> 2 files changed, 85 insertions(+), 4 deletions(-)
> 
> diff --git a/include/trace/events/net.h b/include/trace/events/net.h
> index 00aa72ce0e7c..318307511018 100644
> --- a/include/trace/events/net.h
> +++ b/include/trace/events/net.h
> @@ -117,6 +117,23 @@ DECLARE_EVENT_CLASS(net_dev_template,
> 		__get_str(name), __entry->skbaddr, __entry->len)
> )
> 
> +DECLARE_EVENT_CLASS(net_dev_template_simple,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb),
> +
> +	TP_STRUCT__entry(
> +		__field(void *,	skbaddr)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->skbaddr = skb;
> +	),
> +
> +	TP_printk("skbaddr=%p", __entry->skbaddr)
> +)
> +
> DEFINE_EVENT(net_dev_template, net_dev_queue,
> 
> 	TP_PROTO(struct sk_buff *skb),
> @@ -244,6 +261,48 @@ DEFINE_EVENT(net_dev_rx_verbose_template,
> netif_rx_ni_entry,
> 	TP_ARGS(skb)
> );
> 
> +DEFINE_EVENT(net_dev_template_simple, napi_gro_frags_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> +DEFINE_EVENT(net_dev_template_simple, napi_gro_receive_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> +DEFINE_EVENT(net_dev_template_simple, netif_receive_skb_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> +DEFINE_EVENT(net_dev_template_simple, netif_receive_skb_list_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> +DEFINE_EVENT(net_dev_template_simple, netif_rx_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> +DEFINE_EVENT(net_dev_template_simple, netif_rx_ni_exit,
> +
> +	TP_PROTO(struct sk_buff *skb),
> +
> +	TP_ARGS(skb)
> +);
> +
> #endif /* _TRACE_NET_H */
> 
> /* This part must be outside protection */
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0ffcbdd55fa9..e670ca27e829 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4520,9 +4520,14 @@ static int netif_rx_internal(struct sk_buff *skb)
> 
> int netif_rx(struct sk_buff *skb)
> {
> +	int ret;
> +
> 	trace_netif_rx_entry(skb);
> 
> -	return netif_rx_internal(skb);
> +	ret = netif_rx_internal(skb);
> +	trace_netif_rx_exit(skb);
> +
> +	return ret;
> }
> EXPORT_SYMBOL(netif_rx);
> 
> @@ -4537,6 +4542,7 @@ int netif_rx_ni(struct sk_buff *skb)
> 	if (local_softirq_pending())
> 		do_softirq();
> 	preempt_enable();
> +	trace_netif_rx_ni_exit(skb);
> 
> 	return err;
> }
> @@ -5222,9 +5228,14 @@ static void netif_receive_skb_list_internal(struct
> list_head *head)
>  */
> int netif_receive_skb(struct sk_buff *skb)
> {
> +	int ret;
> +
> 	trace_netif_receive_skb_entry(skb);
> 
> -	return netif_receive_skb_internal(skb);
> +	ret = netif_receive_skb_internal(skb);
> +	trace_netif_receive_skb_exit(skb);
> +
> +	return ret;
> }
> EXPORT_SYMBOL(netif_receive_skb);
> 
> @@ -5247,6 +5258,8 @@ void netif_receive_skb_list(struct list_head *head)
> 	list_for_each_entry(skb, head, list)
> 		trace_netif_receive_skb_list_entry(skb);
> 	netif_receive_skb_list_internal(head);
> +	list_for_each_entry(skb, head, list)
> +		trace_netif_receive_skb_list_exit(skb);
> }
> EXPORT_SYMBOL(netif_receive_skb_list);
> 
> @@ -5634,12 +5647,17 @@ static gro_result_t napi_skb_finish(gro_result_t ret,
> struct sk_buff *skb)
> 
> gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
> {
> +	gro_result_t ret;
> +
> 	skb_mark_napi_id(skb, napi);
> 	trace_napi_gro_receive_entry(skb);
> 
> 	skb_gro_reset_offset(skb);
> 
> -	return napi_skb_finish(dev_gro_receive(napi, skb), skb);
> +	ret = napi_skb_finish(dev_gro_receive(napi, skb), skb);
> +	trace_napi_gro_receive_exit(skb);
> +
> +	return ret;
> }
> EXPORT_SYMBOL(napi_gro_receive);
> 
> @@ -5753,6 +5771,7 @@ static struct sk_buff *napi_frags_skb(struct napi_struct
> *napi)
> 
> gro_result_t napi_gro_frags(struct napi_struct *napi)
> {
> +	gro_result_t ret;
> 	struct sk_buff *skb = napi_frags_skb(napi);
> 
> 	if (!skb)
> @@ -5760,7 +5779,10 @@ gro_result_t napi_gro_frags(struct napi_struct *napi)
> 
> 	trace_napi_gro_frags_entry(skb);
> 
> -	return napi_frags_finish(napi, skb, dev_gro_receive(napi, skb));
> +	ret = napi_frags_finish(napi, skb, dev_gro_receive(napi, skb));
> +	trace_napi_gro_frags_exit(skb);
> +
> +	return ret;
> }
> EXPORT_SYMBOL(napi_gro_frags);
> 
> --
> 2.19.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com