netdev - Re: [PATCH bpf-next 1/5] bpf: cpumap use ptr_ring_consume

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Message-ID: <CAPhsuW4+XAyFTh56wy0bUWDYZj5wQ997t2gGegwkuhroFtR9og@mail.gmail.com>
Date:   Thu, 11 Apr 2019 10:38:09 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        bpf <bpf@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...e.dk>
Subject: Re: [PATCH bpf-next 1/5] bpf: cpumap use ptr_ring_consume_batched

On Thu, Apr 11, 2019 at 4:23 AM Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
>
> On Wed, 10 Apr 2019 16:24:37 -0700
> Song Liu <liu.song.a23@...il.com> wrote:
>
> > >                 /* Feedback loop via tracepoint */
> > > -               trace_xdp_cpumap_kthread(rcpu->map_id, processed, drops, sched);
> > > +               trace_xdp_cpumap_kthread(rcpu->map_id, n, drops, sched);
> >
> > btw: can we do the tracepoint after local_bh_enable()?
>
> I would rather not, as my experience is that this result in strange
> inaccurate readings, because (as comment below says) this is a
> CPU-process-reschedule point.  The test tool reads these values and
> calculate PPS.

Thanks for the explanation!

Song

>
> > >
> > >                 local_bh_enable(); /* resched point, may call do_softirq() */
> > >         }
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer