Re: [PATCH net-next v2] selftests: drv-net: so_txtime: only send test traffic to sch_etf

On Tue, Aug 11, 2026 at 11:27 AM Jakub Kicinski <[email protected]> wrote:
>
> On Tue, 11 Aug 2026 10:29:12 -0400 Willem de Bruijn wrote:
> > I can reproduce after
> >
> > tc qdisc replace dev eth0 root handle pfifo_fast
> > tc qdisc replace dev eth0 root handle 1: pfifo_fast
> >
> > It is not possible to replace tc qdisc replace with a del + add pair,
> > due to "Error: Cannot delete qdisc with handle of zero." on systems
> > with the default 0: qdisc, like my debian trixie.
> >
> > The simplest change, then, is to replace the qdisc twice, first for
> > the kind, then for the handle.
> >
> > @@ -45,17 +45,18 @@ def _qdisc_setup(ifname, qdisc, optargs=""):
> >      """
> >      orig = tc(f"qdisc show dev {ifname} root", json=True)[0].get("kind", None)
> >      defer(tc, f"qdisc replace dev {ifname} root {orig}")
> > +    tc(f"qdisc replace dev {ifname} root {qdisc} {optargs}")
> >      tc(f"qdisc replace dev {ifname} root handle 1: {qdisc} {optargs}")
> >
> > It's an extra replace, so a bit ugly. But avoiding that requires
> > parsing the json for existing handle, branching on that.
> >
> > I'm testing the above some more, will send that if no concerns.
>
> If we are adding an extra command adding del would seem more intuitive?
> But no preference.

Agreed. It's a slightly bigger patch, because it has to be wrapped in a try
block. But its purpose is more obvious.

https://lore.kernel.org/netdev/[email protected]/T/#u

> The issue may be because the ring reconfig test now also sets up qdiscs.
> And we don't have a proper way to restore previous qdisc config in the
> tests. If another test needs to change qdiscs we should probably add
> better infra, but for now I think the simple workaround you suggest is
> best.

Interesting, and that also has special logic to work around default handle 0:

"
    elif root is None or root["kind"] in ("mq", "noqueue"):
        # The auto-attached root mq has handle 0: on any device (real or sim),
        # which the kernel rejects as a qdisc parent. A 0: handle means the mq
        # is the untouched kernel default - no custom child qdiscs can hang off
        # an unaddressable parent - so installing a real handle and restoring
        # the default mq on exit preserves the device's effective policy.
        mq_handle = "1:"
"

Again, sorry for needing an extra fix to what was already a fix :/ We hit a
corner case that even with my testing I just did not reach.