Re: [PATCH 1/2] libbpf: ringbuf: allow to consume up to a certain amount of items

[Andrea Righi <andrea.righi@canonical.com>]

On Tue, Apr 02, 2024 at 10:58:33AM -0700, Andrii Nakryiko wrote:
> On Mon, Apr 1, 2024 at 12:32 AM Andrea Righi <andrea.righi@canonical.com> wrote:
> >
> > In some cases, instead of always consuming all items from ring buffers
> > in a greedy way, we may want to consume up to a certain amount of items,
> > for example when we need to copy items from the BPF ring buffer to a
> > limited user buffer.
> >
> > This change allows to set an upper limit to the amount of items consumed
> > from one or more ring buffers.
> >
> > Link: https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@canonical.com/T
> > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > ---
> >  tools/lib/bpf/ringbuf.c | 29 +++++++++++++++++------------
> >  1 file changed, 17 insertions(+), 12 deletions(-)
> >
> > diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> > index aacb64278a01..81df535040d1 100644
> > --- a/tools/lib/bpf/ringbuf.c
> > +++ b/tools/lib/bpf/ringbuf.c
> > @@ -231,7 +231,7 @@ static inline int roundup_len(__u32 len)
> >         return (len + 7) / 8 * 8;
> >  }
> >
> > -static int64_t ringbuf_process_ring(struct ring *r)
> > +static int64_t ringbuf_process_ring(struct ring *r, int64_t max_items)
> >  {
> >         int *len_ptr, len, err;
> >         /* 64-bit to avoid overflow in case of extreme application behavior */
> > @@ -264,7 +264,14 @@ static int64_t ringbuf_process_ring(struct ring *r)
> >                                                           cons_pos);
> >                                         return err;
> >                                 }
> > -                               cnt++;
> > +                               if (++cnt >= max_items) {
> > +                                       /* update consumer pos and return the
> > +                                        * total amount of items consumed.
> > +                                        */
> > +                                       smp_store_release(r->consumer_pos,
> > +                                                         cons_pos);
> 
> Does this fit on a single line under 100 characters? If yes, please
> keep it as a single line
> 
> but actually it seems cleaner to keep cnt++ intact, let
> smp_store_release() below happen, and then check the exit condition.
> Were you afraid to do unnecessary checks on discarded samples? I
> wouldn't worry about that.

Ok, it makes sense, I'll change it.

> 
> > +                                       goto done;
> > +                               }
> >                         }
> >
> >                         smp_store_release(r->consumer_pos, cons_pos);
> > @@ -281,19 +288,18 @@ static int64_t ringbuf_process_ring(struct ring *r)
> >   */
> >  int ring_buffer__consume(struct ring_buffer *rb)
> >  {
> > -       int64_t err, res = 0;
> > +       int64_t err, res = 0, max_items = INT_MAX;
> 
> I'm wondering if it might be better to have a convention that zero
> means "no limit", which might allow the compiler to eliminate the
> condition in ringbuf_process_ring altogether due to constant
> propagation. WDYT?

Indeed, in this way we won't add any potential overhead to the existing
code that doesn't care about max_items. Will add that.

-Andrea