netdev - Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier support for it

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Date:   Thu, 14 May 2020 13:53:48 -0700
From:   sdf@...gle.com
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier
 support for it

On 05/14, Andrii Nakryiko wrote:
> On Thu, May 14, 2020 at 10:33 AM <sdf@...gle.com> wrote:
> >
> > On 05/13, Andrii Nakryiko wrote:

[...]

> > > + * void bpf_ringbuf_submit(void *data)
> > > + *   Description
> > > + *           Submit reserved ring buffer sample, pointed to by  
> *data*.
> > > + *   Return
> > > + *           Nothing.
> > Even though you mention self-pacing properties, would it still
> > make sense to add some argument to bpf_ringbuf_submit/bpf_ringbuf_output
> > to indicate whether to wake up userspace or not? Maybe something like
> > a threshold of number of outstanding events in the ringbuf after which
> > we do the wakeup? The default 0/1 preserve the existing behavior.
> >
> > The example I can give is a control plane userspace thread that
> > once a second aggregates the events, it doesn't care about millisecond
> > resolution. With the current scheme, I suppose, if BPF generates events
> > every 1ms, the userspace will be woken up 1000 times (if it can keep
> > up). Most of the time, we don't really care and some buffering
> > properties are desired.

> perf buffer has setting like this, and believe me, it's so confusing
> and dangerous, that I wouldn't want this to be exposed. Even though I
> was aware of this behavior, I still had to debug and work-around this
> lack on wakeup few times, it's really-really confusing feature.

> In your case, though, why wouldn't user-space poll data just once a
> second, if it's not interested in getting data as fast as possible?
If I poll once per second I might lose the events if, for some reason,
there is a spike. I really want to have something like: "wakeup
userspace if the ringbuffer fill is over some threshold or
the last wakeup was too long ago". We currently do this via a percpu
cache map. IIRC, you've shared on lsfmmbpf that you do something like
that as well.

So I was thinking how I can use new ringbuff to remove the unneeded
copies and help with the reordering, but I'm a bit concerned about
regressing on the number of wakeups.

Maybe having a flag like RINGBUF_NO_WAKEUP in bpf_ringbuf_submit()
will suffice? And if there is a helper or some way to obtain a
number of unconsumed items, I can implement my own flushing policy.