[PATCH v2 0/3] locking/csd-lock: Report how long a CSD stall lasted

[Breno Leitao <leitao@debian.org>]

The CSD lock debug code says when a CPU stops answering an IPI and when it
finally answers, but the recovery line leaves out how long the stall
lasted, even though csd_lock_wait_toolong() has the timestamp the wait
started from.  On Meta's fleet that line fired 211K times in the last 24
hours, none of them with a duration.

Patch 1 packs the state that csd_lock_wait_toolong() carries across the
polling loop into a struct, as suggested on v1, which gives the timestamps
names and leaves room for another one.  Patch 2 prints how long the lock
was stuck and, when the IPI was re-sent, how long after the last re-send
the target released it.

I've created a simple selftest to test both patches, and I am including
it here, given it might be helpful for whoever is hacking this code.

---
Changes in v2:
- Add patch 1, packing csd_lock_wait_toolong() state into a struct, as
  suggested by Dmitry Ilvokhin.
- Add patch 3, a module that stalls a CPU so the reporting can be exercised.
- Link to v1: https://patch.msgid.link/20260805-csd-stall-duration-v1-1-71134fafe150@debian.org

---
Breno Leitao (3):
      locking/csd-lock: Pack csd_lock_wait_toolong() state into a struct
      locking/csd-lock: Report how long a stuck CSD lock took to recover
      lib/test_csd_lock: Add a module to stall a CPU on a CSD lock

 kernel/smp.c        |  77 +++++++++++++++--------
 lib/Kconfig.debug   |  12 ++++
 lib/Makefile        |   1 +
 lib/test_csd_lock.c | 173 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 237 insertions(+), 26 deletions(-)
---
base-commit: f9a2394a23482bfd330911e9c8295b71724feacd
change-id: 20260805-csd-stall-duration-3385343d7a08

Best regards,
--  
Breno Leitao <leitao@debian.org>