kernel profiling: spinlock_exit consumes 36% CPU time.

邱剑 qj at huawei.com
Tue Oct 7 11:54:11 UTC 2008


Hi, folks,
 
I did kernel profiling when a single thread client sends UDP packets to a
single thread server on the same machine.
 
In the output kernel profile, the first few kernel functions that consumes
the most CPU time are listed below:
 
granularity: each sample hit covers 16 byte(s) for 0.01% of 25.68 seconds

  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 42.4      10.88    10.88        0  100.00%           __mcount [1]
 36.1      20.14     9.26 17937541     0.00     0.00  spinlock_exit [4]
  4.2      21.22     1.08  3145728     0.00     0.00  in_cksum_skip [40]
  1.8      21.68     0.45  7351987     0.00     0.00  generic_copyin [43]
  1.1      21.96     0.29  3146028     0.00     0.00  generic_copyout [48]
  1.0      22.21     0.24  2108904     0.00     0.00  Xint0x80_syscall [3]
  0.8      22.42     0.21  6292131     0.00     0.00  uma_zalloc_arg [46]
  0.8      22.62     0.20  1048576     0.00     0.00  soreceive_generic [9]
  0.7      22.80     0.19  3145852     0.00     0.00  free [47]
  0.6      22.96     0.15  6292172     0.00     0.00  uma_zfree_arg [52]
  0.6      23.10     0.14  5243413     0.00     0.00  generic_bzero [53]
  0.5      23.23     0.14  1048581     0.00     0.00  ip_output [23]
  0.5      23.36     0.13  4221855     0.00     0.00  generic_bcopy [57]
  0.4      23.47     0.11 36865859     0.00     0.00  critical_enter [61]
  0.4      23.57     0.10 36865859     0.00     0.00  critical_exit [62]
  0.4      23.67     0.09 17937541     0.00     0.00  spinlock_enter [63]
  0.4      23.76     0.09  1048582     0.00     0.00  udp_input [21]
  0.3      23.85     0.09  2108904     0.00     0.00  syscall [5]
  0.3      23.93     0.08  1048587     0.00     0.00  ip_input [20]
  0.3      24.00     0.07  2097156     0.00     0.00  getsock [65]
  0.3      24.07     0.07  1048576     0.00     0.00  udp_send [22]
 
It is very strange that spinlock_exit consumes over 36% CPU time while it
seems a very simple function.
 
For clarity, I paste the code of spinlock_exit here:
 
void
spinlock_exit(void)
{
 struct thread *td;
 
 td = curthread;
 critical_exit();
 td->td_md.md_spinlock_count--;
 if (td->td_md.md_spinlock_count == 0)
  intr_restore(td->td_md.md_saved_flags);
}

Since critical_exit consumes only 0.4% CPU time, does this mean the rest of
spinlock_exit consume ~36% CPU time?
 
Am I missing something? Could anybody help me understand this? Many thanks.
 
BTW, the kernel is compiled with SMP and PREEMPTION disabled. The scheduler
is ULE.
 
Best regards,
 
Qiu Jian


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