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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> 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, 

I don't think it is a correct understanding see below.

Current code:
if (m->next != NULL) {
       m->next = NULL;
      m->nb_segs = 1;
}

Proposed code:
if (m->next != NULL)
     m->next = NULL;
if (m->nb_segs != 1)
    m->nb_segs = 1;

So what this patch adds: one more load and compare.
Note that load is from the first mbuf cache line, which
already has to be in the L1 cache by that time.

As I remember the reported slowdown is really tiny.
My vote would be to go ahead with this patch.

> 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
> 
> > -----Original Message-----
> > From: Thomas Monjalon <thomas@monjalon.net>
> > Sent: Tuesday, September 28, 2021 11:29
> > To: Olivier Matz <olivier.matz@6wind.com>; Ali Alnubani
> > <alialnu@nvidia.com>; Slava Ovsiienko <viacheslavo@nvidia.com>
> > Cc: Morten Brørup <mb@smartsharesystems.com>; 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
> >
> > 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.
> >
> >
> >