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SubjectRe: [PATCH net-next] net: skb: fix kfree_skb event output error in perf
On Wed, Aug 24, 2022 at 10:37 PM <menglong8.dong@gmail.com> wrote:
>
> From: Menglong Dong <imagedong@tencent.com>
>
> As Eric reported, the 'reason' field is not presented when trace the
> kfree_skb event by perf:
>
> $ perf record -e skb:kfree_skb -a sleep 10
> $ perf script
> ip_defrag 14605 [021] 221.614303: skb:kfree_skb:
> skbaddr=0xffff9d2851242700 protocol=34525 location=0xffffffffa39346b1
> reason:
>
> The cause seems to be passing kernel address directly to TP_printk(),
> which is not right.

Why ?

It seems this adds an expensive copy of a string that should reside in
rodata section of vmlinux, thus can not disappear...
(Also the ring buffer entry will have a dynamic size ?)

trace_safe_str() is using is_kernel_rodata() which should return true
for drop_reasons[X] ?

$ grep drop_reasons net/core/dropreason_str.c
const char * const drop_reasons[] = {
...



>
> Therefore, fix this by adding a '__string' field to the TP_STRUCT of
> kfree_skb, which is 'reason_str', and passing it to TP_printk().
>
> (Not sure if we should still keep the 'reason' field in
> TP_STRUCT__entry)

Maybe for event/trace filtering purposes ?

>
> Reported-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
> include/trace/events/skb.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/include/trace/events/skb.h b/include/trace/events/skb.h
> index 45264e4bb254..7235554141c3 100644
> --- a/include/trace/events/skb.h
> +++ b/include/trace/events/skb.h
> @@ -24,6 +24,7 @@ TRACE_EVENT(kfree_skb,
> __field(void *, location)
> __field(unsigned short, protocol)
> __field(enum skb_drop_reason, reason)
> + __string(reason_str, drop_reasons[reason])
> ),
>
> TP_fast_assign(
> @@ -31,11 +32,12 @@ TRACE_EVENT(kfree_skb,
> __entry->location = location;
> __entry->protocol = ntohs(skb->protocol);
> __entry->reason = reason;
> + __assign_str(reason_str, drop_reasons[reason]);
> ),
>
> TP_printk("skbaddr=%p protocol=%u location=%p reason: %s",
> __entry->skbaddr, __entry->protocol, __entry->location,
> - drop_reasons[__entry->reason])
> + __get_str(reason_str))
> );
>
> TRACE_EVENT(consume_skb,
> --
> 2.37.2
>

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