[Bug 1470845] Re: systemd on wily desktop generating short lived threads every second in a quiet system

[Martin Pitt]

Unhelpfully this stops after "sudo systemctl daemon-reexec" and even
"sudo systemctl daemon-reload", so this always needs a fresh boot. To
confirm, is that the same on your system?

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Title:
  systemd on wily desktop generating short lived threads every second in
  a quiet system

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  I noticed that systemd on my idle Wily desktop is creating very short
  lived threads at 1Hz.  While these aren't doing much, it still
  consumes power doing wakeups to create these periodic threads.

  Showing thread creation with forkstat:

  $ sudo forkstat 
  Time     Event  PID  Info  Duration Process
  13:43:07 clone     1 parent          /sbin/init splash
  13:43:07 clone  7483 thread          /sbin/init splash
  13:43:07 exit   7483      0    0.001 /sbin/init splash
  13:43:08 clone     1 parent          /sbin/init splash
  13:43:08 clone  7484 thread          /sbin/init splash
  13:43:08 exit   7484      0    0.000 /sbin/init splash
  13:43:10 clone     1 parent          /sbin/init splash
  13:43:10 clone  7485 thread          /sbin/init splash
  13:43:10 exit   7485      0    0.000 /sbin/init splash
  13:43:11 clone     1 parent          /sbin/init splash
  13:43:11 clone  7486 thread          /sbin/init splash
  13:43:11 exit   7486      0    0.000 /sbin/init splash
  13:43:12 clone     1 parent          /sbin/init splash
  13:43:12 clone  7487 thread          /sbin/init splash
  13:43:12 exit   7487      0    0.000 /sbin/init splash
  13:43:13 clone     1 parent          /sbin/init splash
  13:43:13 clone  7488 thread          /sbin/init splash
  13:43:13 exit   7488      0    0.000 /sbin/init splash
  13:43:15 clone     1 parent          /sbin/init splash
  13:43:15 clone  7489 thread          /sbin/init splash
  13:43:15 exit   7489      0    0.000 /sbin/init splash
  13:43:16 clone     1 parent          /sbin/init splash
  13:43:16 clone  7490 thread          /sbin/init splash
  13:43:16 exit   7490      0    0.000 /sbin/init splash
  13:43:17 clone     1 parent          /sbin/init splash
  13:43:17 clone  7491 thread          /sbin/init splash
  13:43:17 exit   7491      0    0.000 /sbin/init splash

  And it's consuming some cycles over time:

  $ sudo perf stat -p 1
  ^C
   Performance counter stats for process id '1':

            7.519868      task-clock (msec)         #    0.000 CPUs utilized          
                  41      context-switches          #    0.005 M/sec                  
                  39      cpu-migrations            #    0.005 M/sec                  
                   3      page-faults               #    0.399 K/sec                  
          12,107,977      cycles                    #    1.610 GHz                    
          10,597,101      stalled-cycles-frontend   #   87.52% frontend cycles idle   
                   0      stalled-cycles-backend    #    0.00% backend  cycles idle   
           2,285,818      instructions              #    0.19  insns per cycle        
                                                    #    4.64  stalled cycles per insn
             457,133      branches                  #   60.790 M/sec                  
              69,444      branch-misses             #   15.19% of all branches        

        46.099593011 seconds time elapsed

  The thread is just doing the following:

  clock_gettime(0x7 /* CLOCK_??? */, {52592, 947682919}) = 0
  read(14, "\1\0\0\0\0\0\0\0", 8)         = 8
  fcntl(30, F_DUPFD_CLOEXEC, 3)           = 15
  ioctl(30, 0xc0189374, 0x7ffeaf311470)   = 0
  fcntl(16, F_GETFD)                      = 0x1 (flags FD_CLOEXEC)
  clone(Process 7466 attached
  child_stack=0x7f97c3580e30, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f97c35819d0, tls=0x7f97c3581700, child_tidptr=0x7f97c35819d0) = 7466
  [pid  7466] set_robust_list(0x7f97c35819e0, 24) = 0
  [pid     1] timerfd_settime(14, TFD_TIMER_ABSTIME, {it_interval={0, 0}, it_value={52594, 197493000}}, NULL <unfinished ...>
  [pid  7466] ioctl(15, 0xc018937c <unfinished ...>
  [pid     1] <... timerfd_settime resumed> ) = 0
  [pid  7466] <... ioctl resumed> , 0x7f97c3580d60) = -1 EAGAIN (Resource temporarily unavailable)
  [pid     1] epoll_wait(4,  <unfinished ...>
  [pid  7466] close(15)                   = 0
  [pid  7466] close(16)                   = 0
  [pid  7466] madvise(0x7f97c2d81000, 8368128, MADV_DONTNEED) = 0
  [pid  7466] _exit(0)                    = ?
  [pid  7466] +++ exited with 0 +++
  <... epoll_wait resumed> {{EPOLLIN, {u32=3, u64=3}}}, 34, -1) = 1

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