linux-kernel - Re: [PATCH net-next] net: skb: fix kfree

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Date:   Thu, 25 Aug 2022 08:31:56 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Miller <davem@...emloft.net>,
        Menglong Dong <imagedong@...cent.com>,
        David Ahern <dsahern@...nel.org>,
        Hao Peng <flyingpeng@...cent.com>,
        Dongli Zhang <dongli.zhang@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: skb: fix kfree_skb event output error in perf

On Wed, Aug 24, 2022 at 10:37 PM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.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@...gle.com>
> Signed-off-by: Menglong Dong <imagedong@...cent.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
>