[lttng-dev] urcu workqueue thread uses 99% of cpu while workqueue is empty

[Minlan Wang]

Hi, Mathieu,
	We are running a CentOS 8.2 os on Intel(R) Xeon(R) CPU E5-2630 v4,
and using the workqueue interfaces in src/workqueue.h in
userspace-rcu-latest-0.12.tar.bz2. 
	Recently, we found the workqueue thread rushes cpu into 99% usage.
After some debuging, we found that the futex in struct urcu_workqueue got
into very big negative value, e.g, -12484; while the qlen, cbs_tail, and
cbs_head suggest that the workqueue is empty.
We add a watchpoint of workqueue->futex in workqueue_thread(), and got this
log when workqueue->futex first get into -2:
...
Old value = -1
New value = 0
0x00007ffff37c1d6d in futex_wake_up (futex=0x55555f74aa40) at workqueue.c:160
160     in workqueue.c
#0  0x00007ffff37c1d6d in futex_wake_up (futex=0x55555f74aa40) at
workqueue.c:160
#1  0x00007ffff37c2737 in wake_worker_thread (workqueue=0x55555f74aa00) at
workqueue.c:324
#2  0x00007ffff37c29fb in urcu_workqueue_queue_work (workqueue=0x55555f74aa00,
work=0x555566e05e00, func=0x7ffff7523c90 <write_dirty_finish>) at
workqueue.c:3
67
#3  0x00007ffff752c520 in aio_complete_cb (ctx=<optimized out>,
iocb=<optimized out>, res=<optimized out>, res2=<optimized out>) at
bio/aio_bio_adapter.c:152
#4  0x00007ffff752c696 in poll_io_complete (arg=0x555562e4f4a0) at
bio/aio_bio_adapter.c:289
#5  0x00007ffff72e6ea5 in start_thread () from /usr/lib64/libpthread.so.0
#6  0x00007ffff415d96d in clone () from /usr/lib64/libc.so.6
[Switching to Thread 0x7fffde3f3700 (LWP 821768)]
Hardware watchpoint 4: -location workqueue->futex

Old value = 0
New value = -1
0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
490     ../include/urcu/uatomic.h: No such file or directory.
#0  0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
#1  workqueue_thread (arg=0x55555f74aa00) at workqueue.c:250
#2  0x00007ffff72e6ea5 in start_thread () from /usr/lib64/libpthread.so.0
#3  0x00007ffff415d96d in clone () from /usr/lib64/libc.so.6
Hardware watchpoint 4: -location workqueue->futex

Old value = -1
New value = -2
0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
490     in ../include/urcu/uatomic.h
#0  0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
#1  workqueue_thread (arg=0x55555f74aa00) at workqueue.c:250
#2  0x00007ffff72e6ea5 in start_thread () from /usr/lib64/libpthread.so.0
#3  0x00007ffff415d96d in clone () from /usr/lib64/libc.so.6
Hardware watchpoint 4: -location workqueue->futex

Old value = -2
New value = -3
0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
490     in ../include/urcu/uatomic.h
#0  0x00007ffff37c2473 in __uatomic_dec (len=4, addr=0x55555f74aa40) at
../include/urcu/uatomic.h:490
#1  workqueue_thread (arg=0x55555f74aa00) at workqueue.c:250
#2  0x00007ffff72e6ea5 in start_thread () from /usr/lib64/libpthread.so.0
#3  0x00007ffff415d96d in clone () from /usr/lib64/libc.so.6
Hardware watchpoint 4: -location workqueue->futex
...

After this, things went into wild, workqueue->futex got into bigger negative
value, and workqueue thread eat up the cpu it is using.
This ends only when workqueue->futex down flew into 0.

Do you have any idea why this is happening, and how to fix it?

B.R
Minlan Wang