Re: [PATCH linux-rcu] docs/litmus-tests: add BPF ringbuf MPSC litmus tests

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, May 29, 2020 at 10:34 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Hi Andrii,
>
> On Thu, May 28, 2020 at 10:50:30PM -0700, Andrii Nakryiko wrote:
> > > [...]
> > > > diff --git a/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus b/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus
> > > > new file mode 100644
> > > > index 000000000000..558f054fb0b4
> > > > --- /dev/null
> > > > +++ b/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus
> > > > @@ -0,0 +1,91 @@
> > > > +C bpf-rb+1p1c+bounded
> > > > +
> > > > +(*
> > > > + * Result: Always
> > > > + *
> > > > + * This litmus test validates BPF ring buffer implementation under the
> > > > + * following assumptions:
> > > > + * - 1 producer;
> > > > + * - 1 consumer;
> > > > + * - ring buffer has capacity for only 1 record.
> > > > + *
> > > > + * Expectations:
> > > > + * - 1 record pushed into ring buffer;
> > > > + * - 0 or 1 element is consumed.
> > > > + * - no failures.
> > > > + *)
> > > > +
> > > > +{
> > > > +     atomic_t dropped;
> > > > +}
> > > > +
> > > > +P0(int *lenFail, int *len1, int *cx, int *px)
> > > > +{
> > > > +     int *rLenPtr;
> > > > +     int rLen;
> > > > +     int rPx;
> > > > +     int rCx;
> > > > +     int rFail;
> > > > +
> > > > +     rFail = 0;
> > > > +
> > > > +     rCx = smp_load_acquire(cx);
> > > > +     rPx = smp_load_acquire(px);
> > >
> > > Is it possible for you to put some more comments around which ACQUIRE is
> > > paired with which RELEASE? And, in general more comments around the reason
> > > for a certain memory barrier and what pairs with what. In the kernel sources,
> > > the barriers needs a comment anyway.
>
> This was the comment earlier that was missed.

Right, I'll follow up extending kernel implementation comments, and
will add some more to litmus tests.

>
> > > > +     if (rCx < rPx) {
> > > > +             if (rCx == 0) {
> > > > +                     rLenPtr = len1;
> > > > +             } else {
> > > > +                     rLenPtr = lenFail;
> > > > +                     rFail = 1;
> > > > +             }
> > > > +
> > > > +             rLen = smp_load_acquire(rLenPtr);
> > > > +             if (rLen == 0) {
> > > > +                     rFail = 1;
> > > > +             } else if (rLen == 1) {
> > > > +                     rCx = rCx + 1;
> > > > +                     smp_store_release(cx, rCx);
> > > > +             }
> > > > +     }
> > > > +}
> > > > +
> > > > +P1(int *lenFail, int *len1, spinlock_t *rb_lock, int *px, int *cx, atomic_t *dropped)
> > > > +{
> > > > +     int rPx;
> > > > +     int rCx;
> > > > +     int rFail;
> > > > +     int *rLenPtr;
> > > > +
> > > > +     rFail = 0;
> > > > +
> > > > +     rCx = smp_load_acquire(cx);
> > > > +     spin_lock(rb_lock);
> > > > +
> > > > +     rPx = *px;
> > > > +     if (rPx - rCx >= 1) {
> > > > +             atomic_inc(dropped);
> > >
> > > Why does 'dropped' need to be atomic if you are always incrementing under a
> > > lock?
> >
> > It doesn't, strictly speaking, but making it atomic in litmus test was
> > just more convenient, especially that I initially also had a lock-less
> > variant of this algorithm.
>
> Ok, that's fine.
>
> > >
> > > > +             spin_unlock(rb_lock);
> > > > +     } else {
> > > > +             if (rPx == 0) {
> > > > +                     rLenPtr = len1;
> > > > +             } else {
> > > > +                     rLenPtr = lenFail;
> > > > +                     rFail = 1;
> > > > +             }
> > > > +
> > > > +             *rLenPtr = -1;
> > >
> > > Clarify please the need to set the length intermittently to -1. Thanks.
> >
> > This corresponds to setting a "busy bit" in kernel implementation.
> > These litmus tests are supposed to be correlated with in-kernel
> > implementation, I'm not sure I want to maintain extra 4 copies of
> > comments here and in kernel code. Especially for 2-producer cases,
> > there are 2 identical P1 and P2, which is unfortunate, but I haven't
> > figured out how to have a re-usable pieces of code with litmus tests
> > :)
>
> I disagree that comments related to memory ordering are optional. IMHO, the
> documentation should be clear from a memory ordering standpoint. After all,
> good Documentation/ always clarifies something / some concept to the reader
> right? :-) Please have mercy on me, I am just trying to learn *your*
> Documentation ;-)

My point was that reading litmus test without also reading ringbuf
implementation is pointless and is harder than necessary. I'll add few
comments to litmus tests, but ultimately I view kernel implementation
as the source of truth and litmus test as a simplified model of it. So
having extensive comments in litmus test is just a maintenance burden
and more chance to get confusing, out-of-sync documentation.

>
> > > > diff --git a/Documentation/litmus-tests/bpf-rb/bpf-rb+2p1c+bounded.litmus b/Documentation/litmus-tests/bpf-rb/bpf-rb+2p1c+bounded.litmus
> [...]
> > > > +P1(int *lenFail, int *len1, spinlock_t *rb_lock, int *px, int *cx, atomic_t *dropped)
> > > > +{
> > > > +     int rPx;
> > > > +     int rCx;
> > > > +     int rFail;
> > > > +     int *rLenPtr;
> > > > +
> > > > +     rFail = 0;
> > > > +     rLenPtr = lenFail;
> > > > +
> > > > +     rCx = smp_load_acquire(cx);
> > > > +     spin_lock(rb_lock);
> > > > +
> > > > +     rPx = *px;
> > > > +     if (rPx - rCx >= 1) {
> > > > +             atomic_inc(dropped);
> > > > +             spin_unlock(rb_lock);
> > > > +     } else {
> > > > +             if (rPx == 0) {
> > > > +                     rLenPtr = len1;
> > > > +             } else if (rPx == 1) {
> > > > +                     rLenPtr = len1;
> > > > +             } else {
> > > > +                     rLenPtr = lenFail;
> > > > +                     rFail = 1;
> > > > +             }
> > > > +
> > > > +             *rLenPtr = -1;
> > > > +             smp_store_release(px, rPx + 1);
> > > > +
> > > > +             spin_unlock(rb_lock);
> > > > +
> > > > +             smp_store_release(rLenPtr, 1);
> > >
> > > I ran a test replacing the last 2 statements above with the following and it
> > > still works:
> > >
> > >                 spin_unlock(rb_lock);
> > >                 WRITE_ONCE(*rLenPtr, 1);
> > >
> > > Wouldn't you expect the test to catch an issue? The spin_unlock is already a
> > > RELEASE barrier.
> >
> > Well, apparently it's not an issue and WRITE_ONCE would work as well
> > :) My original version actually used WRITE_ONCE here. See [0] and
> > discussion in [1] after which I removed all the WRITE_ONCE/READ_ONCE
> > in favor of store_release/load_acquire for consistency.
> >
> >   [0] https://patchwork.ozlabs.org/project/netdev/patch/20200513192532.4058934-3-andriin@fb.com/
> >   [1] https://patchwork.ozlabs.org/project/netdev/patch/20200513192532.4058934-2-andriin@fb.com/
>
> Huh. So you are replacing the test to use WRITE_ONCE instead? Why did you
> favor the acquire/release memory barriers over the _ONCE annotations, if that
> was not really needed then?

I replaced WRITE_ONCE with store_release. There was a request on
initial version to keep it simple and use store_release/load_acquire
pairings consistently and not mix up WRITE_ONCE and load_acquire, so
that's what I did. As I mentioned elsewhere, this might not be the
weakest possible set of orderings and we might improve that, but it
seems to work well.

>
> > > Suggestion: It is hard to review the patch because it is huge, it would be
> > > good to split this up into 4 patches for each of the tests. But upto you :)
> >
> > Those 4 files are partial copies of each other, not sure splitting
> > them actually would be easier. If anyone else thinks the same, though,
> > I'll happily split.
>
> I personally disagree. It would be much easier IMHO to review 4 different
> files since some of them are also quite dissimilar. I frequently keep jumping
> between diffs to find a different file and it makes the review that much
> harder. But anything the LKMM experts decide in this regard is acceptable to me :)
>
> thanks,
>
>  - Joel
>