RE: [PATCH] ring: add cache guard after ring elements table

["Morten Brørup" <mb@smartsharesystems.com>]

+CC: Stable maintainers, so you can decide on backporting.

> > > > Added cache guard after the table holding the ring elements, to
> avoid
> > > > false sharing conflicts caused by next-line hardware prefetchers
> when
> > > > accessing elements at the end of the ring table.
> > >
> > > I don't see any harm with it, and in theory it might help in some
> > > cases...
> > > Though I wonder how real is that problem?
> > > Did you ever observe such contention to happen?
> >
> > I never observed a problem with this.
> > The risk of contention depends on what is allocated in the memory
> after the ring.
> > Which is application specific.
> >
> > It seems like a purely theoretical issue, but should be fixed anyway,
> to eliminate
> > that risk.
> 
> Ok, as I said I see no harm with it.
> Should we document this change somewhere? RN or PG?

We don't want the release notes overflowing with minor details.
IMO, this change is below the threshold for what people might care about.
People interested in the detailed changes between releases should read the git log.

Also, I don't think it's worth backporting, because I consider it unlikely to have any real effect.
In the context of backporting, it could be considered a performance improvement rather than a bug fix.

> Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> 
> > >
> > > > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > > > ---
> > > >  lib/ring/rte_ring.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/lib/ring/rte_ring.c b/lib/ring/rte_ring.c
> > > > index f10050a1c4..9ccc62cd42 100644
> > > > --- a/lib/ring/rte_ring.c
> > > > +++ b/lib/ring/rte_ring.c
> > > > @@ -73,8 +73,11 @@ rte_ring_get_memsize_elem(unsigned int esize,
> > > unsigned
> > > > int count)
> > > >  		return -EINVAL;
> > > >  	}
> > > >
> > > > +	static_assert(sizeof(struct rte_ring) ==
> > > > RTE_CACHE_LINE_ROUNDUP(sizeof(struct rte_ring)),
> > > > +			"Size of struct rte_ring not cache line
> aligned");
> > > >  	sz = sizeof(struct rte_ring) + (ssize_t)count * esize;
> > > >  	sz = RTE_ALIGN(sz, RTE_CACHE_LINE_SIZE);
> > > > +	sz += RTE_CACHE_GUARD_LINES * RTE_CACHE_LINE_SIZE;
> > > >  	return sz;
> > > >  }
> > > >
> > > > --
> > > > 2.43.0