[Bug 1272438] Re: kwin 100% of cpu, second session

Wed Jan 28 14:47:45 UTC 2015

** Description changed:

  Steps to reproduce
  Login as user1
  Login as user2
  Press Ctrl+Alt+F7 (to switch to first user)

  kwin from second user starts to eat 100% of cpu.

  Here is my investigation of what is going on

  1)  ====== GDB  ======
  I attached to running (and eating 100% of cpu) kwin, paused ("ctrl+c" command) and run ("continue" command) kwin several times and watched output of info threads command to figure out what thread is eating 100%.
  What I got:

  (gdb) info threads
    Id   Target Id         Frame
    3    Thread 0x7fa21ecb6700 (LWP 25128) "QProcessManager" 0x00007fa23f3c8de3 in select () at ../sysdeps/unix/syscall-template.S:81
    2    Thread 0x7fa21c9c6700 (LWP 25131) "kwin" pthread_cond_wait@@GLIBC_2.3.2 ()
      at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  * 1    Thread 0x7fa23fbfe7c0 (LWP 25123) "kwin" 0x00007fa236ea7f84 in __GI___pthread_mutex_lock (mutex=0x2520918) at pthread_mutex_lock.c:54
  (gdb) continue

  // continue, wait some time, ctrl+c

  (gdb) info threads
    Id   Target Id         Frame
    3    Thread 0x7fa21ecb6700 (LWP 25128) "QProcessManager" 0x00007fa23f3c8de3 in select () at ../sysdeps/unix/syscall-template.S:81
    2    Thread 0x7fa21c9c6700 (LWP 25131) "kwin" pthread_cond_wait@@GLIBC_2.3.2 ()
      at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  * 1    Thread 0x7fa23fbfe7c0 (LWP 25123) "kwin" 0x00007fa23f3d1957 in __libc_recv (fd=9, buf=0x2520974, n=4096, flags=-1, flags at entry=0)
      at ../sysdeps/unix/sysv/linux/x86_64/recv.c:33

  // continue, wait some time, ctrl+c

  (gdb) info threads
    Id   Target Id         Frame
    3    Thread 0x7fa21ecb6700 (LWP 25128) "QProcessManager" 0x00007fa23f3c8de3 in select () at ../sysdeps/unix/syscall-template.S:81
    2    Thread 0x7fa21c9c6700 (LWP 25131) "kwin" pthread_cond_wait@@GLIBC_2.3.2 ()
      at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  * 1    Thread 0x7fa23fbfe7c0 (LWP 25123) "kwin" 0x00007fa23f3c8de3 in select () at ../sysdeps/unix/syscall-template.S:81

  and so on...

  As you can see first thread is really doing something.

  Now, when I know thread number I can see backtrace:
  First backtrace
  [Switching to thread 1 (Thread 0x7fa23fbfe7c0 (LWP 25123))]
  #0  0x00007fa238b11303 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
  (gdb) backtrace
  #0  0x00007fa238b11303 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
  #1  0x00007fa2396d05ef in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #2  0x00007fa2396d08e5 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #3  0x00007fa2396d5e5b in QCoreApplication::exec() () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #4  0x00007fa23f719956 in kdemain () from /usr/lib/kde4/libkdeinit/libkdeinit4_kwin.so
  #5  0x00007fa23f2f7de5 in __libc_start_main (main=0x4006d0, argc=3, ubp_av=0x7ffffe010848, init=<optimized out>, fini=<optimized out>,
      rtld_fini=<optimized out>, stack_end=0x7ffffe010838) at libc-start.c:260
  #6  0x00000000004006fe in _start ()

  // continue, wait some time, ctrl+c

  And second one (after "continue" and "ctrl+c")
  (gdb) backtrace
  #0  0x00007fa23f3c8de3 in select () at ../sysdeps/unix/syscall-template.S:81
  #1  0x00007fa2396fc33b in qt_safe_select(int, fd_set*, fd_set*, fd_set*, timeval const*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #2  0x00007fa239701904 in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timeval*) ()
     from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #3  0x00007fa239701d02 in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
     from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #4  0x00007fa238b112f6 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
  #5  0x00007fa2396d05ef in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #6  0x00007fa2396d08e5 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #7  0x00007fa2396d5e5b in QCoreApplication::exec() () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
  #8  0x00007fa23f719956 in kdemain () from /usr/lib/kde4/libkdeinit/libkdeinit4_kwin.so
  #9  0x00007fa23f2f7de5 in __libc_start_main (main=0x4006d0, argc=3, ubp_av=0x7ffffe010848, init=<optimized out>, fini=<optimized out>,
      rtld_fini=<optimized out>, stack_end=0x7ffffe010838) at libc-start.c:260
  #10 0x00000000004006fe in _start ()
  (gdb)

  2)  ====== strace  ======
  Now lets see which syscalls are called, I ran this command
  strace -p 25123 -s 1000 -o strace1.txt
  And in 1-2 seconds I got file (with 50000 lines) which contains ONLY these messages:

  select(29, [4 5 9 10 14 28], [], [], {0, 0}) = 0 (Timeout)
  recvfrom(9, 0x2520974, 4096, 0, 0, 0)   = -1 EAGAIN (Resource temporarily unavailable)
  select(29, [4 5 9 10 14 28], [], [], {0, 0}) = 0 (Timeout)
  recvfrom(9, 0x2520974, 4096, 0, 0, 0)   = -1 EAGAIN (Resource temporarily unavailable)
  select(29, [4 5 9 10 14 28], [], [], {0, 0}) = 0 (Timeout)
  recvfrom(9, 0x2520974, 4096, 0, 0, 0)   = -1 EAGAIN (Resource temporarily unavailable)
  select(29, [4 5 9 10 14 28], [], [], {0, 0}) = 0 (Timeout)
  recvfrom(9, 0x2520974, 4096, 0, 0, 0)   = -1 EAGAIN (Resource temporarily unavailable)
  select(29, [4 5 9 10 14 28], [], [], {0, 0}) = 0 (Timeout)

  Notice first argument of recvfrom - 9, this fd we already saw at gdb "info threads":
  * 1    Thread 0x7fa23fbfe7c0 (LWP 25123) "kwin" 0x00007fa23f3d1957 in __libc_recv (fd=9, buf=0x2520974, n=4096, flags=-1,

  ProblemType: Bug
  DistroRelease: Ubuntu 13.10
  Package: kde-window-manager 4:4.11.3-0ubuntu0.1
  ProcVersionSignature: Ubuntu 3.11.0-15.23-generic 3.11.10
  Uname: Linux 3.11.0-15-generic x86_64
  NonfreeKernelModules: nvidia
  ApportVersion: 2.12.5-0ubuntu2.2
  Architecture: amd64
  Date: Fri Jan 24 21:20:31 2014
  InstallationDate: Installed on 2013-11-11 (74 days ago)
  InstallationMedia: Kubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
  MarkForUpload: True
  SourcePackage: kde-workspace
  UpgradeStatus: No upgrade log present (probably fresh install)
+ 
+ Arch forum posts:
+ https://bbs.archlinux.org/viewtopic.php?id=186904
+ https://bbs.archlinux.org/viewtopic.php?id=173030

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https://bugs.launchpad.net/bugs/1272438

Title:
  kwin 100% of cpu, second session

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