netdev - Re: [PATCH net-next] net-sysfs: display two backlog queue len separately

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Date:   Mon, 13 Mar 2023 08:59:11 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jason Xing <kerneljasonxing@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] net-sysfs: display two backlog queue len separately

On Mon, Mar 13, 2023 at 6:16 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Mon, Mar 13, 2023 at 8:34 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Sat, Mar 11, 2023 at 7:18 AM 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>
> > > ---
> > >  net/core/net-procfs.c | 17 ++++++++++++-----
> > >  1 file changed, 12 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/net/core/net-procfs.c b/net/core/net-procfs.c
> > > index 1ec23bf8b05c..97a304e1957a 100644
> > > --- a/net/core/net-procfs.c
> > > +++ b/net/core/net-procfs.c
> > > @@ -115,10 +115,14 @@ static int dev_seq_show(struct seq_file *seq, void *v)
> > >         return 0;
> > >  }
> > >
> > > -static u32 softnet_backlog_len(struct softnet_data *sd)
> > > +static u32 softnet_input_pkt_queue_len(struct softnet_data *sd)
> > >  {
> > > -       return skb_queue_len_lockless(&sd->input_pkt_queue) +
> > > -              skb_queue_len_lockless(&sd->process_queue);
> > > +       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 struct softnet_data *softnet_get_online(loff_t *pos)
> > > @@ -169,12 +173,15 @@ 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);
> > > +                  0,   /* was len of two backlog queues */
> >
> > You can not pretend the sum is zero, some user space tools out there
> > would be fooled.
> >
> > > +                  (int)seq->index,
> > > +                  softnet_input_pkt_queue_len(sd), softnet_process_queue_len(sd));
> > >         return 0;
> > >  }
> > >
> > > --
> > > 2.37.3
> > >
> >
> > In general I would prefer we no longer change this file.
>
> Fine. Since now, let this legacy file be one part of history.
>
> >
> > Perhaps add a tracepoint instead ?
>
> Thanks, Eric. It's one good idea. It seems acceptable if we only need
> to trace two separate backlog queues where it can probably hit the
> limit, say, in the enqueue_to_backlog().


Note that enqueue_to_backlog() already uses a specific kfree_skb_reason() reason
(SKB_DROP_REASON_CPU_BACKLOG) so existing infrastructure should work just fine.


>
> Similarly I decide to write another two tracepoints of time_squeeze
> and budget_squeeze which I introduced to distinguish from time_squeeze
> as the below link shows:
> https://lore.kernel.org/lkml/CAL+tcoAwodpnE2NjMLPhBbmHUvmKMgSykqx0EQ4YZaQHjrx0Hw@mail.gmail.com/.
> For that change, any suggestions are deeply welcome :)
>

For your workloads to hit these limits enough for you to be worried,
it looks like you are not using any scaling stuff documented
in Documentation/networking/scaling.rst