linux-kernel - Re: [PATCH v2 net-next 1/2] net-sysfs: display two backlog queue len separately

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Date:   Tue, 14 Mar 2023 07:59:27 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jason Xing <kerneljasonxing@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
        john.fastabend@...il.com, stephen@...workplumber.org,
        simon.horman@...igine.com, sinquersw@...il.com,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH v2 net-next 1/2] net-sysfs: display two backlog queue len separately

On Mon, Mar 13, 2023 at 8:06 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> Sometimes we need to know which one of backlog queue can be exactly
> long enough to cause some latency when debugging this part is needed.
> Thus, we can then separate the display of both.
>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> v2: keep the total len of backlog queues untouched as Eric said
> Link: https://lore.kernel.org/lkml/20230311151756.83302-1-kerneljasonxing@gmail.com/
> ---
>  net/core/net-procfs.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/net/core/net-procfs.c b/net/core/net-procfs.c
> index 1ec23bf8b05c..2809b663e78d 100644
> --- a/net/core/net-procfs.c
> +++ b/net/core/net-procfs.c
> @@ -115,10 +115,19 @@ static int dev_seq_show(struct seq_file *seq, void *v)
>         return 0;
>  }
>
> +static u32 softnet_input_pkt_queue_len(struct softnet_data *sd)
> +{
> +       return skb_queue_len_lockless(&sd->input_pkt_queue);
> +}
> +
> +static u32 softnet_process_queue_len(struct softnet_data *sd)
> +{
> +       return skb_queue_len_lockless(&sd->process_queue);
> +}
> +
>  static u32 softnet_backlog_len(struct softnet_data *sd)
>  {
> -       return skb_queue_len_lockless(&sd->input_pkt_queue) +
> -              skb_queue_len_lockless(&sd->process_queue);
> +       return softnet_input_pkt_queue_len(sd) + softnet_process_queue_len(sd);

Reading these variables twice might lead to inconsistency that can
easily be avoided.

I would suggest you cache the values,

u32 len1 = softnet_input_pkt_queue_len(sd);
u32 len2 = softnet_process_queue_len(sd);



>  }
>
>  static struct softnet_data *softnet_get_online(loff_t *pos)
> @@ -169,12 +178,15 @@ static int softnet_seq_show(struct seq_file *seq, void *v)
>          * mapping the data a specific CPU
>          */
>         seq_printf(seq,
> -                  "%08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x\n",
> +                  "%08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x "
> +                  "%08x %08x\n",
>                    sd->processed, sd->dropped, sd->time_squeeze, 0,
>                    0, 0, 0, 0, /* was fastroute */
>                    0,   /* was cpu_collision */
>                    sd->received_rps, flow_limit_count,
> -                  softnet_backlog_len(sd), (int)seq->index);
> +                  softnet_backlog_len(sd),     /* keep it untouched */
                    len1 + len2.

> +                  (int)seq->index,
> +                  softnet_input_pkt_queue_len(sd), softnet_process_queue_len(sd));
               len1,  len2);

>         return 0;
>  }
>
> --
> 2.37.3
>