Re: [dpdk-dev] [dpdk-stable] [PATCH v4] mbuf: fix reset on mbuf free

[Ali Alnubani <alialnu@oss.nvidia.com>]

Hi Olivier,

I wanted to retest the patch on latest main, but it no longer applies, could you please rebase it?

Thanks,
Ali

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Tuesday, September 28, 2021 12:40 PM
> To: Slava Ovsiienko <viacheslavo@nvidia.com>; NBU-Contact-Thomas
> Monjalon <thomas@monjalon.net>; Olivier Matz <olivier.matz@6wind.com>;
> Ali Alnubani <alialnu@nvidia.com>
> Cc: dev@dpdk.org; David Marchand <david.marchand@redhat.com>; Alexander
> Kozyrev <akozyrev@nvidia.com>; Ferruh Yigit <ferruh.yigit@intel.com>;
> zhaoyan.chen@intel.com; Andrew Rybchenko
> <andrew.rybchenko@oktetlabs.ru>; Ananyev, Konstantin
> <konstantin.ananyev@intel.com>; Ajit Khaparde
> <ajit.khaparde@broadcom.com>; jerinj@marvell.com
> Subject: RE: [dpdk-dev] [dpdk-stable] [PATCH v4] mbuf: fix reset on mbuf free
> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Slava Ovsiienko
> > Sent: Tuesday, 28 September 2021 11.01
> >
> > Hi,
> >
> > I've re-read the entire thread.
> > If I understand correctly, the root problem was (in initial patch):
> >
> > >   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
> > >
> > The proposed fix was to ALWAYS set next and nb_seg fields on
> > mbuf_free(), regardless next field content. That would perform
> > unconditional write to mbuf, and might affect the configurations,
> > where are no multi- segment packets at al. mbuf_free() is "backbone"
> > API, it is used by all cases, all scenaries are affected.
> >
> > As far as I know, the current approach for nb_seg field - it contains
> > other value than 1 only in the first mbuf , for the following
> > segments,  it should not be considered at all (only the first segment
> > fields are valid), and it is supposed to contain 1, as it was
> > initially allocated from the pool.
> >
> > In the example above the problem was introduced by
> > rte_pktmbuf_chain(). Could we consider fixing the rte_pktmbuf_chain()
> > (used in potentially fewer common sceneries)  instead of touching the
> > extremely common rte_mbuf_free() ?
> >
> > With best regards,
> > Slava
> 
> Great idea, Slava!
> 
> Changing the invariant for 'nb_segs', so it must be 1, except in the first segment
> of a segmented packet.
> 
> Thinking further about it, perhaps we can achieve even higher performance by a
> minor additional modification: Use 0 instead of 1? Or offset 'nb_segs' by -1, so it
> reflects the number of additional segments?
> 
> And perhaps combining the invariants for 'nb_segs' and 'next' could provide even
> more performance improvements. I don't know, just sharing a thought.
> 
> Anyway, I vote for fixing the bug. One way or the other!
> 
> -Morten
> 
> >
> > > -----Original Message-----
> > > From: Thomas Monjalon <thomas@monjalon.net>
> > > Sent: Tuesday, September 28, 2021 11:29
> > >
> > > Follow-up again:
> > > We have added a note in 21.08, we should fix it in 21.11.
> > > If there are no counter proposal, I suggest applying this patch, no
> > matter the
> > > performance regression.
> > >
> > >
> > > 30/07/2021 16:54, Thomas Monjalon:
> > > > 30/07/2021 16:35, Morten Brørup:
> > > > > > From: Olivier Matz [mailto:olivier.matz@6wind.com]
> > > > > > Sent: Friday, 30 July 2021 14.37
> > > > > >
> > > > > > Hi Thomas,
> > > > > >
> > > > > > On Sat, Jul 24, 2021 at 10:47:34AM +0200, Thomas Monjalon
> > wrote:
> > > > > > > What's the follow-up for this patch?
> > > > > >
> > > > > > Unfortunatly, I still don't have the time to work on this
> > > > > > topic
> > yet.
> > > > > >
> > > > > > In my initial tests, in our lab, I didn't notice any
> > performance
> > > > > > regression, but Ali has seen an impact (0.5M PPS, but I don't
> > know
> > > > > > how much in percent).
> > > > > >
> > > > > >
> > > > > > > 19/01/2021 15:04, Slava Ovsiienko:
> > > > > > > > Hi, All
> > > > > > > >
> > > > > > > > Could we postpose this patch at least to rc2? We would
> > > > > > > > like
> > to
> > > > > > conduct more investigations?
> > > > > > > >
> > > > > > > > With best regards, Slava
> > > > > > > >
> > > > > > > > From: Olivier Matz <olivier.matz@6wind.com>
> > > > > > > > > On Mon, Jan 18, 2021 at 05:52:32PM +0000, Ali Alnubani
> > wrote:
> > > > > > > > > > Hi,
> > > > > > > > > > (Sorry had to resend this to some recipients due to
> > mail
> > > > > > > > > > server
> > > > > > problems).
> > > > > > > > > >
> > > > > > > > > > Just confirming that I can still reproduce the
> > regression
> > > > > > > > > > with
> > > > > > single core and
> > > > > > > > > 64B frames on other servers.
> > > > > > > > >
> > > > > > > > > Many thanks for the feedback. Can you please detail what
> > is
> > > > > > > > > the
> > > > > > amount of
> > > > > > > > > performance loss in percent, and confirm the test case?
> > (I
> > > > > > suppose it is
> > > > > > > > > testpmd io forward).
> > > > > > > > >
> > > > > > > > > Unfortunatly, I won't be able to spend a lot of time on
> > this
> > > > > > > > > soon
> > > > > > (sorry for
> > > > > > > > > that). So I see at least these 2 options:
> > > > > > > > >
> > > > > > > > > - postpone the patch again, until I can find more time
> > > > > > > > > to
> > analyze
> > > > > > > > >   and optimize
> > > > > > > > > - apply the patch if the performance loss is acceptable
> > > > > > > > > compared
> > > > > > to
> > > > > > > > >   the added value of fixing a bug
> > > > > > > > >
> > > > > > > [...]
> > > > > >
> > > > > > Statu quo...
> > > > > >
> > > > > > Olivier
> > > > > >
> > > > >
> > > > > The decision should be simple:
> > > > >
> > > > > Does the DPDK project support segmented packets?
> > > > > If yes, then apply the patch to fix the bug!
> > > > >
> > > > > If anyone seriously cares about the regression it introduces,
> > optimization
> > > patches are welcome later. We shouldn't wait for it.
> > > >
> > > > You're right, but the regression is flagged to a 4-years old
> > > > patch, that's why I don't consider it as urgent.
> > > >
> > > > > If the patch is not applied, the documentation must be updated
> > > > > to
> > > mention that we are releasing DPDK with a known bug: that segmented
> > > packets are handled incorrectly in the scenario described in this
> > patch.
> > > >
> > > > Yes, would be good to document the known issue, no matter how old
> > it
> > > > is.
> > > >
> > > > > Generally, there could be some performance to gain by not
> > supporting
> > > segmented packets at all, as a compile time option. But that is a
> > different
> > > discussion.
> > >
> > >
> > >
> >