[Crash-utility] Running idle threads show wrong CPU numbers

[Michael Holzheu]

On Wed, 2010-02-10 at 10:08 -0500, Dave Anderson wrote:
> ----- "Michael Holzheu" <holzheu at linux.vnet.ibm.com> wrote:
> 
> > Hi again,
> 
> > > When I change get_smp_cpus() to return "get_highest_cpu_online() + 1" I
> > > see five swapper idle tasks when using "ps". The problem I now have is
> > > that I have to provide a backtrace for the offline cpus. But the offline
> > > CPUs do not have any stack on s390. Is there a way to tell crash that
> > > there is no backtrace available? Probably I overlooked something...
> > 
> > Ok, I think I got it now. In case of an offline CPU, I will use
> > "task_struct_thread_ksp" like I do it for non active tasks.
> > 
> > When I do that I get for the swapper tasks with the offline CPUs:
> > 
> > PID: 0      TASK: 18d38340          CPU: 2   COMMAND: "swapper"
> >  #0 [18d3feb8] ret_from_fork at 117e12
> > 
> > PID: 0      TASK: 18d40440          CPU: 3   COMMAND: "swapper"
> >  #0 [18d47eb8] ret_from_fork at 117e12
> 
> I'm not why you should do anything.  The cpu is offline and for all
> practical purposes it doesn't exist, so why bother?

Because you can do a "bt" on the swapper task with the offline CPU.
Then s390x_get_stack_frame() is called where I figure out the stack
pointer and instruction address. In that function I check if the task is
currently running on a CPU and in that case I get the information from
the associated s390 lowcore, where the registers are stored in case of a
dump. If the task is not running I get the information from the thread
struct.

> The patch I have queued just uses get_highest_cpu_online()+1 and
> does nothing else.  But I only tested it on a live system, and
> any backtrace attempt on the offlined swapper task just shows
> (active).  What happens when you do a "bt -a" with a dumpfile?

It shows all swapper tasks (online and offline), but I get errors for
the backtrace for the offline CPUs.

The attached patch would solve the problem (and eliminate most of the
probably redundant s390(x)_has_cpu() function.


With this patch "ps" shows:

   PID    PPID  CPU       TASK        ST  %MEM     VSZ    RSS  COMM
>     0      0   0       800ef0       RU   0.0       0      0  [swapper]
>     0      0   1      18d30240      RU   0.0       0      0  [swapper]
>     0      0   2      18d38340      RU   0.0       0      0  [swapper]
>     0      0   3      18d40440      RU   0.0       0      0  [swapper]
>     0      0   4      18d48540      RU   0.0       0      0  [swapper]
      1      0   1      18d18040      IN   0.2    2244   1020  init
...

And "bt -a" shows:

PID: 0      TASK: 800ef0            CPU: 0   COMMAND: "swapper"
 LOWCORE INFO:
  -psw      : 0x0706000180000000 0x0000000000115564
  -function : vtime_stop_cpu at 115564
  -prefix   : 0x18d28000
  -cpu timer: 0x7fff00c1 0x00c584ef
...

PID: 0      TASK: 18d30240          CPU: 1   COMMAND: "swapper"
 LOWCORE INFO:
  -psw      : 0x0706000180000000 0x0000000000115564
  -function : vtime_stop_cpu at 115564
...

PID: 0      TASK: 18d38340          CPU: 2   COMMAND: "swapper"
 #0 [18d3feb8] ret_from_fork at 117e12
...

PID: 0      TASK: 18d40440          CPU: 3   COMMAND: "swapper"
 #0 [18d47eb8] ret_from_fork at 117e12
...

PID: 0      TASK: 18d48540          CPU: 4   COMMAND: "swapper"
 LOWCORE INFO:
  -psw      : 0x0706000180000000 0x0000000000115564
  -function : vtime_stop_cpu at 115564
  -prefix   : 0x1416a000

Michael
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