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

[Mathieu Desnoyers]

----- On Jun 13, 2022, at 11:55 PM, Minlan Wang wangminlan at szsandstone.com wrote:

> Hi, Mathieu,
>	We are running a CentOS 8.2 os on Intel(R) Xeon(R) CPU E5-2630 v4,

Also, can you provide more information about which exact Linux kernel version
you are using ?

Thanks,

Mathieu

> 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com