[PATCH 00/17] Backport rt/deadline crash and the ardous story of FUTEX_UNLOCK_PI to 4.4

[Henrik Austad]

From: Henrik Austad <haustad@xxxxxxxxx>

Short story:

The following patches are needed on a 4.4 kernel to avoid
Oops in the scheduler when a sched_rr and sched_deadline task contends
on the same futex (with PI).

Longer story:

On one of our arm64 systems, we occasionally crash with an Oops in the
scheduler with the following backtrace.

[<ffffffc0000ee398>] enqueue_task_dl+0x1f0/0x420
[<ffffffc0000d0f14>] activate_task+0x7c/0x90
[<ffffffc0000edbdc>] push_dl_task+0x164/0x1c8
[<ffffffc0000edc60>] push_dl_tasks+0x20/0x30
[<ffffffc0000cc00c>] __balance_callback+0x44/0x68
[<ffffffc000d2c018>] __schedule+0x6f0/0x728
[<ffffffc000d2c278>] schedule+0x78/0x98
[<ffffffc000d2e76c>] __rt_mutex_slowlock+0x9c/0x108
[<ffffffc000d2e9d0>] rt_mutex_slowlock+0xd8/0x198
[<ffffffc0000f7f28>] rt_mutex_timed_futex_lock+0x30/0x40
[<ffffffc00012c1a8>] futex_lock_pi+0x200/0x3b0
[<ffffffc00012cf84>] do_futex+0x1c4/0x550
[<ffffffc00012d92c>] compat_SyS_futex+0x10c/0x138
[<ffffffc00008504c>] __sys_trace_return+0x0/0x4

This seems to be the same bug Xuneli Pang triggered and fixed in
e96a7705e7d3 "sched/rtmutex/deadline: Fix a PI crash for deadline
tasks". As noted by Peter Zijlstra in the previous attempt, this fix
requires a few other patches, most notably the FUTEX_UNLOCK_PI series
[1]

Testing this on a dual-core VM I have not been able to reproduce the
same crash, but pi_stress (part of the rt-test suite) reveals that
vanilla 4.4.162 behaves rather badly with a mix of deadline and
sched_(rr|fifo) tasks:

time pi_stress --rr --mlockall --sched id=high,policy=deadline,runtime=100000,deadline=200000,period=200000
Starting PI Stress Test
Number of thread groups: 1
Duration of test run: infinite
Number of inversions per group: unlimited
     Admin thread SCHED_RR priority 4
1 groups of 3 threads will be created
      High thread SCHED_DEADLINE runtime 100000 deadline 200000 period 200000
       Med thread SCHED_RR priority 2
       Low thread SCHED_RR priority 1
Current Inversions: 141627
WATCHDOG triggered: group 0 is deadlocked!
reporter stopping due to watchdog event
Stopping test
Terminated

real    0m26.291s
user    0m0.148s
sys     0m18.819s

With this series applied, the test ran for ~4.5 hours and again for 129
minutes (when I remembered to time it) before crashing:

time pi_stress --rr --mlockall --sched id=high,policy=deadline,runtime=100000,deadline=200000,period=200000
Starting PI Stress Test
Number of thread groups: 1
Duration of test run: infinite
Number of inversions per group: unlimited
     Admin thread SCHED_RR priority 4
1 groups of 3 threads will be created
      High thread SCHED_DEADLINE runtime 100000 deadline 200000 period 200000
       Med thread SCHED_RR priority 2
       Low thread SCHED_RR priority 1
Current Inversions: 51985223
WATCHDOG triggered: group 0 is deadlocked!
reporter stopping due to watchdog event
Stopping test
Terminated

real    129m38.807s
user    0m59.084s
sys     109m53.666s


So clearly not perfect, but a *lot* better.

The same series on our vendor-4.4 kernel moves pi_stress up from ~30
seconds before deadlock up to the same level as the VM (the test is
still going as of this writing).

I suspect other users of 4.4 would benefit from having these patches
backported, so tag them for stable. I assume 4.9 and 4.14 could benefit
as well, but I have not had time to look into those.

1) https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1359667.html

Peter Zijlstra (13):
  futex: Cleanup variable names for futex_top_waiter()
  futex: Use smp_store_release() in mark_wake_futex()
  futex: Remove rt_mutex_deadlock_account_*()
  futex,rt_mutex: Provide futex specific rt_mutex API
  futex: Change locking rules
  futex: Cleanup refcounting
  futex: Rework inconsistent rt_mutex/futex_q state
  futex: Pull rt_mutex_futex_unlock() out from under hb->lock
  futex,rt_mutex: Introduce rt_mutex_init_waiter()
  futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock()
  futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()
  futex: Futex_unlock_pi() determinism
  futex: Drop hb->lock before enqueueing on the rtmutex

Thomas Gleixner (2):
  rtmutex: Make wait_lock irq safe
  futex: Rename free_pi_state() to put_pi_state()

Xunlei Pang (2):
  rtmutex: Deboost before waking up the top waiter
  sched/rtmutex/deadline: Fix a PI crash for deadline tasks

 include/linux/init_task.h       |   1 +
 include/linux/sched.h           |   2 +
 include/linux/sched/rt.h        |   1 +
 kernel/fork.c                   |   1 +
 kernel/futex.c                  | 532 ++++++++++++++++++++++++++--------------
 kernel/locking/rtmutex-debug.c  |   9 -
 kernel/locking/rtmutex-debug.h  |   3 -
 kernel/locking/rtmutex.c        | 406 ++++++++++++++++++------------
 kernel/locking/rtmutex.h        |   2 -
 kernel/locking/rtmutex_common.h |  24 +-
 kernel/sched/core.c             |   2 +
 11 files changed, 620 insertions(+), 363 deletions(-)

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2.7.4