On Wed, 20 Jan 2021 11:14:37 +0200 Ido Schimmel wrote:
> On Tue, Jan 19, 2021 at 02:22:55PM -0800, Jakub Kicinski wrote:
> > On Sun, 17 Jan 2021 10:02:18 +0200 Ido Schimmel wrote:
> > > From: Ido Schimmel <ido...@nvidia.com>
> > >
> > > The RED qdisc currently supports two qevents: "early_drop" and "mark". The
> > > filters added to the block bound to the "early_drop" qevent are executed
> > > on
> > > packets for which the RED algorithm decides that they should be
> > > early-dropped. The "mark" filters are similarly executed on ECT packets
> > > that are marked as ECN-CE (Congestion Encountered).
> > >
> > > A previous patchset has offloaded "early_drop" filters on Spectrum-2 and
> > > later, provided that the classifier used is "matchall", that the action
> > > used is either "trap" or "mirred", and a handful or further limitations.
> >
> > For early_drop trap or mirred makes obvious sense, no explanation
> > needed.
> >
> > But for marked as a user I'd like to see a _copy_ of the packet,
> > while the original continues on its marry way to the destination.
> > I'd venture to say that e.g. for a DCTCP deployment mark+trap is
> > unusable, at least for tracing, because it distorts the operation
> > by effectively dropping instead of marking.
> >
> > Am I reading this right?
>
> You get a copy of the packet as otherwise it will create a lot of
> problems (like you wrote).
Hm, so am I missing some background on semantics on TC_ACT_TRAP?
Or perhaps you use a different action code?
AFAICT the code in the kernel is:
struct sk_buff *tcf_qevent_handle(...
case TC_ACT_STOLEN:
case TC_ACT_QUEUED:
case TC_ACT_TRAP:
__qdisc_drop(skb, to_free);
*ret = __NET_XMIT_STOLEN;
return NULL;
Having TRAP mean DROP makes sense for filters, but in case of qevents
shouldn't they be a no-op?
Looking at sch_red looks like TRAP being a no-op would actually give us
the expected behavior.
> > If that is the case and you really want to keep the mark+trap
> > functionality - I feel like at least better documentation is needed.
> > The current two liner should also be rewritten, quoting from patch 1:
> >
> > > * - ``ecn_mark``
> > > - ``drop``
> > > - Traps ECN-capable packets that were marked with CE (Congestion
> > > Encountered) code point by RED algorithm instead of being dropped
> >
> > That needs to say that the trap is for datagrams trapped by a qevent.
> > Otherwise "Traps ... instead of being dropped" is too much of a
> > thought-shortcut, marked packets are not dropped.
> >
> > (I'd also think that trap is better documented next to early_drop,
> > let's look at it from the reader's perspective)
>
> How about:
>
> "Traps a copy of ECN-capable packets that were marked with CE
I think "Traps copies" or "Traps the copy of .. packet"?
I'm not a native speaker but there seems to be a grammatical mix here.
> (Congestion Encountered) code point by RED algorithm instead of being
> dropped. The trap is enabled by attaching a filter with action 'trap' to
... instead of those copies being dropped.
> the 'mark' qevent of the RED qdisc."
>
> In addition, this output:
>
> $ devlink trap show pci/0000:06:00.0 trap ecn_mark
> pci/0000:06:00.0:
> name ecn_mark type drop generic true action trap group buffer_drops
>
> Can be converted to:
>
> $ devlink trap show pci/0000:06:00.0 trap ecn_mark
> pci/0000:06:00.0:
> name ecn_mark type drop generic true action mirror group buffer_drops
>
> "mirror: The packet is forwarded by the underlying device and a copy is sent
> to
> the CPU."
>
> In this case the action is static and you cannot change it.
Oh yes, that's nice, I thought mirror in traps means mirror to another
port. Are there already traps which implement the mirroring / trapping
a clone? Quick grep yields nothing of substance.
