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 <m...@smartsharesystems.com> > Sent: Tuesday, September 28, 2021 12:40 PM > To: Slava Ovsiienko <viachesl...@nvidia.com>; NBU-Contact-Thomas > Monjalon <tho...@monjalon.net>; Olivier Matz <olivier.m...@6wind.com>; > Ali Alnubani <alia...@nvidia.com> > Cc: dev@dpdk.org; David Marchand <david.march...@redhat.com>; Alexander > Kozyrev <akozy...@nvidia.com>; Ferruh Yigit <ferruh.yi...@intel.com>; > zhaoyan.c...@intel.com; Andrew Rybchenko > <andrew.rybche...@oktetlabs.ru>; Ananyev, Konstantin > <konstantin.anan...@intel.com>; Ajit Khaparde > <ajit.khapa...@broadcom.com>; jer...@marvell.com > Subject: RE: [dpdk-dev] [dpdk-stable] [PATCH v4] mbuf: fix reset on mbuf free > > > From: dev [mailto:dev-boun...@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 <tho...@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.m...@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.m...@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. > > > > > > > > > > >
