[PATCH net 0/3] net/sched: taprio: fix software schedule livelocks

[Junjie Cao <junjie.cao@intel.com>]

advance_sched() livelocks the owning CPU in two independent ways:
schedules with entry intervals below the cost of servicing one hrtimer
expiry pass validation, because virtual devices inflate the link speed
behind the frame-length minimum, and a valid schedule that falls
behind - delayed timer, starved CPU, stepped clock - replays its whole
backlog one entry per expiry from hrtimer context.

Neither fix covers the other case. With only bounded catch-up, a 700ns
single-entry schedule on veth is still admitted and sustains ~1M timer
irqs/s on a release build. With only the interval floor, a stepped
clock still replays the backlog. Clamping the next expiry into the
future at runtime, as tested on one of the reproducer buckets in 2025
[1], keeps the stall detector quiet but leaves the sub-microsecond
schedule admitted, the CPU servicing an expiry every few microseconds
for the lifetime of the qdisc, and the gates drifting off the
configured timeline with every clamped expiry.

Patch 2 extends the patch generated by syzkaller's patching workflow
[2] to exempt txtime-assist, which never arms the per-entry hrtimer.
syzbot tested the series against the reproducers of all three known
buckets on net.git; tags on patch 2.

[1] https://lore.kernel.org/all/20250729010657.3326-1-hdanton@sina.com/
[2] https://lore.kernel.org/all/afe041f6-ef7d-4434-b2d0-096be49b5bcb@mail.kernel.org/


Junjie Cao (2):
  net/sched: taprio: catch up in bounded time when the schedule falls
    behind
  selftests/tc-testing: taprio: add case for the software minimum
    interval

Uladzislau Zhauniarovich (1):
  net/sched: taprio: enforce a minimum interval for software schedules

 net/sched/sch_taprio.c                        | 80 +++++++++++++++++--
 .../tc-testing/tc-tests/qdiscs/taprio.json    | 22 +++++
 2 files changed, 97 insertions(+), 5 deletions(-)

-- 
2.43.0