[Qemu-devel] [Bug 1581334] Re: qemu + librbd takes high %sy cpu under high random io workload

[chenwqin]

Here are gdb oupout with librbd1-dbg and librados2-dbg.

---------------------------------------------------------
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007ff8cf8dddff in ppoll () from /lib/x86_64-linux-gnu/libc.so.6

Thread 522 (Thread 0x7ff8ccb56700 (LWP 4959)):
#0  0x00007ff8cf8e2809 in syscall () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ff8cf198012 in ?? () from /usr/lib/x86_64-linux-gnu/liblttng-ust.so.0
#2  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#3  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 521 (Thread 0x7ff8cc355700 (LWP 4960)):
#0  0x00007ff8cf8e2809 in syscall () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ff8cf198012 in ?? () from /usr/lib/x86_64-linux-gnu/liblttng-ust.so.0
#2  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#3  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 520 (Thread 0x7ff8cbb54700 (LWP 4961)):
#0  0x00007ff8cf8e2809 in syscall () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x0000000000833d00 in futex_wait (ev=0x1222e04 <rcu_call_ready_event>, 
val=4294967295) at util/qemu-thread-posix.c:292
#2  0x0000000000833e8e in qemu_event_wait (ev=0x1222e04 <rcu_call_ready_event>) 
at util/qemu-thread-posix.c:399
#3  0x0000000000849382 in call_rcu_thread (opaque=0x0) at util/rcu.c:250
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 519 (Thread 0x7ff8cab1c700 (LWP 4974)):
#0  0x00007ff8cfbb508f in pthread_cond_wait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d17a563b in ceph::log::Log::entry (this=0x1643a60) at 
log/Log.cc:353
#2  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#3  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 518 (Thread 0x7ff8c9b8d700 (LWP 4975)):
#0  0x00007ff8cfbb5438 in pthread_cond_timedwait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d1535acb in WaitUntil (when=..., mutex=..., this=0x1692bd8) at 
./common/Cond.h:71
#2  WaitInterval (interval=..., mutex=..., cct=<optimized out>, this=0x1692bd8) 
at ./common/Cond.h:79
#3  CephContextServiceThread::entry (this=0x1692b60) at 
common/ceph_context.cc:96
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6


Thread 517 (Thread 0x7ff8c938c700 (LWP 4976)):
#0  0x00007ff8cfbb5438 in pthread_cond_timedwait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d15125c1 in WaitUntil (when=..., mutex=..., this=0x1694978) at 
common/Cond.h:71
#2  SafeTimer::timer_thread (this=0x1694968) at common/Timer.cc:118
#3  0x00007ff8d1512efd in SafeTimerThread::entry (this=<optimized out>) at 
common/Timer.cc:38
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 516 (Thread 0x7ff8c8b8b700 (LWP 4977)):
#0  0x00007ff8cfbb508f in pthread_cond_wait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d1696b70 in Wait (mutex=..., this=0x1693f50) at ./common/Cond.h:55
#2  DispatchQueue::entry (this=0x1693ee8) at msg/simple/DispatchQueue.cc:196
#3  0x00007ff8d16c65fd in DispatchQueue::DispatchThread::entry (this=<optimized 
out>) at msg/simple/DispatchQueue.h:103
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 515 (Thread 0x7ff8c838a700 (LWP 4978)):
#0  0x00007ff8cfbb508f in pthread_cond_wait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d1699ba6 in Wait (mutex=..., this=0x16940f0) at ./common/Cond.h:55
#2  DispatchQueue::run_local_delivery (this=0x1693ee8) at 
msg/simple/DispatchQueue.cc:114
#3  0x00007ff8d16c66dd in DispatchQueue::LocalDeliveryThread::entry 
(this=<optimized out>) at msg/simple/DispatchQueue.h:117
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6

Thread 514 (Thread 0x7ff8c7b89700 (LWP 4979)):
#0  0x00007ff8cfbb508f in pthread_cond_wait@@GLIBC_2.3.2 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007ff8d16bde99 in Wait (mutex=..., this=0x1694358) at ./common/Cond.h:55
#2  SimpleMessenger::reaper_entry (this=0x1693d20) at 
msg/simple/SimpleMessenger.cc:211
#3  0x00007ff8d16c6cdd in SimpleMessenger::ReaperThread::entry (this=<optimized 
out>) at msg/simple/SimpleMessenger.h:195
#4  0x00007ff8cfbb10a4 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ff8cf8e687d in clone () from /lib/x86_64-linux-gnu/libc.so.6
........
........
........
(all the log in attachment: debug.log)
---------------------------------------------------------------------------

** Attachment added: "gdb thread apply all bt with librbd1-dbg and 
librados2-dbg"
   
https://bugs.launchpad.net/qemu/+bug/1581334/+attachment/4662277/+files/debug.log

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https://bugs.launchpad.net/bugs/1581334

Title:
  qemu + librbd takes high %sy cpu under high random io workload

Status in Ceph:
  New
Status in Linux:
  New
Status in QEMU:
  New

Bug description:
  I got an IO problem. When running Qemu + ceph(use librbd), and do a random IO 
benchmark or some high load random IO test, it will exhaust all my host cpu on 
%sy cpu.
  It doesn’t happen all the time, but when it appear it will reproduce every 
time I start a random IO benchmark(test with Fio).
  And the only way to fix the problem is shutdown my vm and start it, but the 
problem will happen again with high random IO load.

  Some information:
      Vendor      : HP
      Product     : HP ProLiant BL460c Gen9
      Kernel      : 3.16.0-4-amd64
      Disto       : Debian
      Version     : 8.4
      Arch        : amd64
      Qemu        : 2.1 ~ 2.6 (Yes, I already test the latest qemu2.6 version, 
but still got the problem)
      Ceph        : Hammer 0.94.5
      Librbd      : 0.94.5 ~ 10.2 (I rebuild librbd with ceph 10.2 source code, 
but the problem still here)
      Qemu config : cache=none
      Qemu cpu&mem: 4core, 8GB

  How can i reproduce the problem?

  while :; do bash randwrite.sh ; sleep 3600; done >test.log 2>&1 &
  (Sleep 3600 is the key to reproduce my problem. I don’t known how long sleep 
suit for reproduce, but one hour sleep is enough. the problem will easy 
reproduce after a long sleep, if i keep benchmark running without sleep, i 
can't reproduce it)

  My randwrite.sh script
  ----------------------------------------------
  #!/bin/sh
  sync
  echo 3 > /proc/sys/vm/drop_caches

  FILENAME=/dev/vdc
  RUNTIME=100
  BLOCKSIZE=4K
  IOENGINE=libaio
  RESULTFILE=fio-randwrite.log
  IODEPTH=32
  RAMP_TIME=5
  SIZE=100G

  fio --numjobs 10 --norandommap --randrepeat=0 --readwrite=randwrite 
--ramp_time=$RAMP_TIME --bs=$BLOCKSIZE --runtime=$RUNTIME --iodepth=$IODEPTH 
--filename=$FILENAME --ioengine=$IOENGINE --direct=1 --name=iops_randwrite 
--group_reporting  | tee $RESULTFILE
  ----------------------------------------------

  What happened after the problem appear?
  my vm will got huge IOPS drop. In my case, it will drop from 15000 IOPS to 
3500 IOPS. And other thing, my host cpu will exhaust on %sy. Top output like 
this.

  Qemu Fio benchmark
  ----------------------------------------------------
  Tasks: 284 total,   2 running, 282 sleeping,   0 stopped,   0 zombie
  %Cpu0  : 11.8 us, 66.7 sy,  0.0 ni, 21.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu1  : 12.7 us, 64.9 sy,  0.0 ni, 22.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu2  : 13.7 us, 64.5 sy,  0.0 ni, 21.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu3  : 13.2 us, 64.1 sy,  0.0 ni, 22.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu4  : 11.7 us, 65.4 sy,  0.0 ni, 22.8 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu5  : 13.2 us, 64.4 sy,  0.0 ni, 22.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu6  : 12.4 us, 65.1 sy,  0.0 ni, 22.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu7  : 13.6 us, 63.8 sy,  0.0 ni, 22.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu8  :  9.8 us, 73.0 sy,  0.0 ni, 17.2 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu9  :  7.8 us, 74.5 sy,  0.0 ni, 17.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu10 :  6.0 us, 81.4 sy,  0.0 ni,  6.6 id,  0.0 wa,  0.0 hi,  6.0 si,  0.0 
st
  %Cpu11 :  8.4 us, 79.5 sy,  0.0 ni,  8.8 id,  0.0 wa,  0.0 hi,  3.4 si,  0.0 
st
  %Cpu12 :  7.6 us, 80.7 sy,  0.0 ni,  7.0 id,  0.0 wa,  0.0 hi,  4.7 si,  0.0 
st
  %Cpu13 :  7.4 us, 79.9 sy,  0.0 ni,  7.7 id,  0.0 wa,  0.0 hi,  5.0 si,  0.0 
st
  %Cpu14 :  9.8 us, 75.4 sy,  0.0 ni, 11.4 id,  0.0 wa,  0.0 hi,  3.4 si,  0.0 
st
  %Cpu15 :  6.7 us, 80.1 sy,  0.0 ni, 10.1 id,  0.0 wa,  0.0 hi,  3.0 si,  0.0 
st
  %Cpu16 :  9.2 us, 69.2 sy,  0.0 ni, 17.5 id,  0.0 wa,  0.0 hi,  4.1 si,  0.0 
st
  %Cpu17 :  9.9 us, 66.6 sy,  0.0 ni, 20.1 id,  0.0 wa,  0.0 hi,  3.4 si,  0.0 
st
  %Cpu18 : 16.6 us, 49.0 sy,  0.0 ni, 34.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu19 : 16.7 us, 46.4 sy,  0.0 ni, 36.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu20 : 13.0 us, 50.8 sy,  0.0 ni, 36.1 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu21 : 18.9 us, 46.2 sy,  0.0 ni, 34.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu22 : 12.1 us, 52.9 sy,  0.0 ni, 35.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu23 : 15.9 us, 47.6 sy,  0.0 ni, 36.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu24 :  6.7 us, 62.0 sy,  0.0 ni, 31.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu25 :  7.6 us, 63.7 sy,  0.0 ni, 28.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu26 :  8.1 us, 75.8 sy,  0.0 ni, 16.1 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu27 :  6.7 us, 73.6 sy,  0.0 ni, 19.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu28 :  9.2 us, 74.3 sy,  0.0 ni, 16.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu29 :  8.2 us, 73.3 sy,  0.0 ni, 18.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu30 :  4.4 us, 73.1 sy,  0.0 ni, 22.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  %Cpu31 :  7.5 us, 69.6 sy,  0.0 ni, 22.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 
st
  KiB Mem:  13217662+total,  3721572 used, 12845504+free,   283228 buffers
  KiB Swap:  4194300 total,        0 used,  4194300 free.  2242976 cached Mem

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
  30349 root      20   0 25.381g 499892  20640 R  2495  0.4 119:11.98 
qemu-system-x86

  Anything I do?
  I use perf top, profile to debug the problem. It show me that something like 
thread deadlock problem. Any I test QEMU with kernel RBD, it work fine.
  Here are the perf top output on host.
  ---------------------------------------------------------------
     PerfTop:   12393 irqs/sec  kernel:87.3%  exact:  0.0% [4000Hz cycles],  
(all, 32 CPUs)
  
-------------------------------------------------------------------------------

      75.25%  [kernel]            [k] _raw_spin_lock
       1.17%  [kernel]            [k] futex_wait_setup
       0.86%  libc-2.19.so        [.] malloc
       0.58%  [kernel]            [k] futex_wake
       0.55%  libc-2.19.so        [.] 0x00000000000ea96f
       0.41%  [kernel]            [k] native_write_msr_safe
  ---------------------------------------------------------------

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