[dpdk-dev] [PATCH] mbuf: fix reset on mbuf free

[Morten Brørup]

> From: Olivier Matz [mailto:olivier.matz at 6wind.com]
> Sent: Friday, November 6, 2020 11:05 AM
> 
> On Fri, Nov 06, 2020 at 09:50:45AM +0100, Morten Brørup wrote:
> > > From: Olivier Matz [mailto:olivier.matz at 6wind.com]
> > > Sent: Friday, November 6, 2020 9:21 AM
> > >
> > > On Fri, Nov 06, 2020 at 08:52:58AM +0100, Morten Brørup wrote:
> > > > > From: Ananyev, Konstantin [mailto:konstantin.ananyev at intel.com]
> > > > > Sent: Friday, November 6, 2020 12:55 AM
> > > > >
> > > > > > > > > > > > > >> Hi Olivier,
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >>> m->nb_seg must be reset on mbuf free
> whatever
> > > the
> > > > > value
> > > > > > > of m->next,
> > > > > > > > > > > > > >>> because it can happen that m->nb_seg is !=
> 1.
> > > For
> > > > > > > instance in this
> > > > > > > > > > > > > >>> case:
> > > > > > > > > > > > > >>>
> > > > > > > > > > > > > >>>   m1 = rte_pktmbuf_alloc(mp);
> > > > > > > > > > > > > >>>   rte_pktmbuf_append(m1, 500);
> > > > > > > > > > > > > >>>   m2 = rte_pktmbuf_alloc(mp);
> > > > > > > > > > > > > >>>   rte_pktmbuf_append(m2, 500);
> > > > > > > > > > > > > >>>   rte_pktmbuf_chain(m1, m2);
> > > > > > > > > > > > > >>>   m0 = rte_pktmbuf_alloc(mp);
> > > > > > > > > > > > > >>>   rte_pktmbuf_append(m0, 500);
> > > > > > > > > > > > > >>>   rte_pktmbuf_chain(m0, m1);
> > > > > > > > > > > > > >>>
> > > > > > > > > > > > > >>> As rte_pktmbuf_chain() does not reset
> nb_seg in
> > > the
> > > > > > > initial m1
> > > > > > > > > > > > > >>> segment (this is not required), after this
> code
> > > the
> > > > > > > mbuf chain
> > > > > > > > > > > > > >>> have 3 segments:
> > > > > > > > > > > > > >>>   - m0: next=m1, nb_seg=3
> > > > > > > > > > > > > >>>   - m1: next=m2, nb_seg=2
> > > > > > > > > > > > > >>>   - m2: next=NULL, nb_seg=1
> > > > > > > > > > > > > >>>
> > > > > > > > > > > > > >>> Freeing this mbuf chain will not restore
> > > nb_seg=1
> > > > > in
> > > > > > > the second
> > > > > > > > > > > > > >>> segment.
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> Hmm, not sure why is that?
> > > > > > > > > > > > > >> You are talking about freeing m1, right?
> > > > > > > > > > > > > >> rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > > > > > > > > > > > > >> {
> > > > > > > > > > > > > >> 	...
> > > > > > > > > > > > > >> 	if (m->next != NULL) {
> > > > > > > > > > > > > >>                         m->next = NULL;
> > > > > > > > > > > > > >>                         m->nb_segs = 1;
> > > > > > > > > > > > > >>                 }
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> m1->next != NULL, so it will enter the if()
> > > block,
> > > > > > > > > > > > > >> and will reset both next and nb_segs.
> > > > > > > > > > > > > >> What I am missing here?
> > > > > > > > > > > > > >> Thinking in more generic way, that change:
> > > > > > > > > > > > > >>  -		if (m->next != NULL) {
> > > > > > > > > > > > > >>  -			m->next = NULL;
> > > > > > > > > > > > > >>  -			m->nb_segs = 1;
> > > > > > > > > > > > > >>  -		}
> > > > > > > > > > > > > >>  +		m->next = NULL;
> > > > > > > > > > > > > >>  +		m->nb_segs = 1;
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Ah, sorry. I oversimplified the example and
> now
> > > it
> > > > > does
> > > > > > > not
> > > > > > > > > > > > > > show the issue...
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > The full example also adds a split() to break
> the
> > > > > mbuf
> > > > > > > chain
> > > > > > > > > > > > > > between m1 and m2. The kind of thing that
> would
> > > be
> > > > > done
> > > > > > > for
> > > > > > > > > > > > > > software TCP segmentation.
> > > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > > > > If so, may be the right solution is to care
> about
> > > > > nb_segs
> > > > > > > > > > > > > when next is set to NULL on split? Any place
> when
> > > next
> > > > > is
> > > > > > > set
> > > > > > > > > > > > > to NULL. Just to keep the optimization in a
> more
> > > > > generic
> > > > > > > place.
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > > The problem with that approach is that there are
> > > already
> > > > > > > several
> > > > > > > > > > > > existing split() or trim() implementations in
> > > different
> > > > > DPDK-
> > > > > > > based
> > > > > > > > > > > > applications. For instance, we have some in
> > > 6WINDGate. If
> > > > > we
> > > > > > > force
> > > > > > > > > > > > applications to set nb_seg to 1 when resetting
> next,
> > > it
> > > > > has
> > > > > > > to be
> > > > > > > > > > > > documented because it is not straightforward.
> > > > > > > > > > >
> > > > > > > > > > > I think it is better to go that way.
> > > > > > > > > > > From my perspective it seems natural to reset
> nb_seg at
> > > > > same
> > > > > > > time
> > > > > > > > > > > we reset next, otherwise inconsistency will occur.
> > > > > > > > > >
> > > > > > > > > > While it is not explicitly stated for nb_segs, to me
> it
> > > was
> > > > > clear
> > > > > > > that
> > > > > > > > > > nb_segs is only valid in the first segment, like for
> many
> > > > > fields
> > > > > > > (port,
> > > > > > > > > > ol_flags, vlan, rss, ...).
> > > > > > > > > >
> > > > > > > > > > If we say that nb_segs has to be valid in any
> segments,
> > > it
> > > > > means
> > > > > > > that
> > > > > > > > > > chain() or split() will have to update it in all
> > > segments,
> > > > > which
> > > > > > > is not
> > > > > > > > > > efficient.
> > > > > > > > >
> > > > > > > > > Why in all?
> > > > > > > > > We can state that nb_segs on non-first segment should
> > > always
> > > > > equal
> > > > > > > 1.
> > > > > > > > > As I understand in that case, both split() and chain()
> have
> > > to
> > > > > > > update nb_segs
> > > > > > > > > only for head mbufs, rest ones will remain untouched.
> > > > > > > >
> > > > > > > > Well, anyway, I think it's strange to have a constraint
> on m-
> > > > > >nb_segs
> > > > > > > for
> > > > > > > > non-first segment. We don't have that kind of constraints
> for
> > > > > other
> > > > > > > fields.
> > > > > > >
> > > > > > > True, we don't. But this is one of the fields we consider
> > > critical
> > > > > > > for proper work of mbuf alloc/free mechanism.
> > > > > > >
> > > > > >
> > > > > > I am not sure that requiring m->nb_segs == 1 on non-first
> > > segments
> > > > > will provide any benefits.
> > > > >
> > > > > It would make this patch unneeded.
> > > > > So, for direct, non-segmented mbufs  pktmbuf_free() will remain
> > > write-
> > > > > free.
> > > >
> > > > I see. Then I agree with Konstantin that alternative solutions
> should
> > > be considered.
> > > >
> > > > The benefit regarding free()'ing non-segmented mbufs - which is a
> > > very common operation - certainly outweighs the cost of requiring
> > > split()/chain() operations to set the new head mbuf's nb_segs = 1.
> > > >
> > > > Nonetheless, the bug needs to be fixed somehow.
> > > >
> > > > If we can't come up with a better solution that doesn't break the
> > > ABI, we are forced to accept the patch.
> > > >
> > > > Unless the techboard accepts to break the ABI in order to avoid
> the
> > > performance cost of this patch.
> > >
> > > Did someone notice a performance drop with this patch?
> > > On my side, I don't see any regression on a L3 use case.
> >
> > I am afraid that the DPDK performance regression tests are based on
> TX immediately following RX, so cache misses in TX may go by unnoticed
> because RX warmed up the cache for TX already. And similarly for RX
> reusing mbufs that have been warmed up by the preceding free() at TX.
> >
> > Please consider testing the performance difference with the mbuf
> being completely cold at TX, and going completely cold again before
> being reused for RX.
> >
> > >
> > > Let's sumarize: splitting a mbuf chain and freeing it causes
> subsequent
> > > mbuf
> > > allocation to return a mbuf which is not correctly initialized.
> There
> > > are 2
> > > options to fix it:
> > >
> > > 1/ change the mbuf free function (this patch)
> > >
> > >    - m->nb_segs would behave like many other field: valid in the
> first
> > >      segment, ignored in other segments
> > >    - may impact performance (suspected)
> > >
> > > 2/ change all places where a mbuf chain is split, or trimmed
> > >
> > >    - m->nb_segs would have a specific behavior: count the number of
> > >      segments in the first mbuf, should be 1 in the last segment,
> > >      ignored in other ones.
> > >    - no code change in mbuf library, so no performance impact
> > >    - need to patch all places where we do a mbuf split or trim.
> From
> > > afar,
> > >      I see at least mbuf_cut_seg_ofs() in DPDK. Some external
> > > applications
> > >      may have to be patched (for instance, I already found 3 places
> in
> > >      6WIND code base without a deep search).
> > >
> > > In my opinion, 1/ is better, except we notice a significant
> > > performance,
> > > because the (implicit) behavior is unchanged.
> > >
> > > Whatever the solution, some documentation has to be added.
> > >
> > > Olivier
> > >
> >
> > Unfortunately, I don't think that anything but the first option will
> go into 20.11 and stable releases of older versions, so I stand by my
> acknowledgment of the patch.
> 
> If we are affraid about 20.11 performance (it is legitimate, few days
> before the release), we can target 21.02. After all, everybody lives
> with this bug since 2017, so there is no urgency. If accepted and well
> tested, it can be backported in stable branches.

+1

Good thinking, Olivier!