Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598

[Vladislav Zolotarov <vladz@cloudius-systems.com>]

On Sep 11, 2015 7:07 PM, "Richardson, Bruce" <bruce.richardson@intel.com>
wrote:
>
>
>
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Vladislav Zolotarov
> > Sent: Friday, September 11, 2015 5:04 PM
> > To: Avi Kivity
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above
1
> > for all NICs but 82598
> >
> > On Sep 11, 2015 6:43 PM, "Avi Kivity" <avi@cloudius-systems.com> wrote:
> > >
> > > On 09/11/2015 06:12 PM, Vladislav Zolotarov wrote:
> > >>
> > >>
> > >> On Sep 11, 2015 5:55 PM, "Thomas Monjalon"
> > >> <thomas.monjalon@6wind.com>
> > wrote:
> > >> >
> > >> > 2015-09-11 17:47, Avi Kivity:
> > >> > > On 09/11/2015 05:25 PM, didier.pallard wrote:
> > >> > > > On 08/25/2015 08:52 PM, Vlad Zolotarov wrote:
> > >> > > >>
> > >> > > >> Helin, the issue has been seen on x540 devices. Pls., see a
> > chapter
> > >> > > >> 7.2.1.1 of x540 devices spec:
> > >> > > >>
> > >> > > >> A packet (or multiple packets in transmit segmentation) can
> > >> > > >> span
> > any
> > >> > > >> number of
> > >> > > >> buffers (and their descriptors) up to a limit of 40 minus
> > >> > > >> WTHRESH minus 2 (see Section 7.2.3.3 for Tx Ring details and
> > >> > > >> section Section 7.2.3.5.1
> > for
> > >> > > >> WTHRESH
> > >> > > >> details). For best performance it is recommended to minimize
> > >> > > >> the number of buffers as possible.
> > >> > > >>
> > >> > > >> Could u, pls., clarify why do u think that the maximum number
> > >> > > >> of
> > data
> > >> > > >> buffers is limited by 8?
> > >> > > >>
> > >> > > >> thanks,
> > >> > > >> vlad
> > >> > > >
> > >> > > > Hi vlad,
> > >> > > >
> > >> > > > Documentation states that a packet (or multiple packets in
> > >> > > > transmit
> > >> > > > segmentation) can span any number of buffers (and their
> > >> > > > descriptors) up to a limit of 40 minus WTHRESH minus 2.
> > >> > > >
> > >> > > > Shouldn't there be a test in transmit function that drops
> > >> > > > properly
> > the
> > >> > > > mbufs with a too large number of segments, while incrementing a
> > >> > > > statistic; otherwise transmit
> > function
> > >> > > > may be locked by the faulty packet without notification.
> > >> > > >
> > >> > >
> > >> > > What we proposed is that the pmd expose to dpdk, and dpdk expose
> > >> > > to
> > the
> > >> > > application, an mbuf check function.  This way applications that
> > >> > > can generate complex packets can verify that the device will be
> > >> > > able to process them, and applications that only generate simple
> > >> > > mbufs can
> > avoid
> > >> > > the overhead by not calling the function.
> > >> >
> > >> > More than a check, it should be exposed as a capability of the
port.
> > >> > Anyway, if the application sends too much segments, the driver must
> > >> > drop it to avoid hang, and maintain a dedicated statistic counter
> > >> > to
> > allow
> > >> > easy debugging.
> > >>
> > >> I agree with Thomas - this should not be optional. Malformed packets
> > should be dropped. In the icgbe case it's a very simple test - it's a
> > single branch per packet so i doubt that it could impose any measurable
> > performance degradation.
> > >>
> > >>
> > >
> > > A drop allows the application no chance to recover.  The driver must
> > either provide the ability for the application to know that it cannot
> > accept the packet, or it must fix it up itself.
> >
> > An appropriate statistics counter would be a perfect tool to detect such
> > issues. Knowingly sending a packet that will cause a HW to hang is not
> > acceptable.
>
> I would agree. Drivers should provide a function to query the max number
of
> segments they can accept and the driver should be able to discard any
packets
> exceeding that number, and just track it via a stat.

+1

>
> /Bruce