[Crash-utility] Running idle threads show wrong CPU numbers
----- Forwarded Message -----
From: "Dave Anderson" <anderson(a)redhat.com>
----- "Michael Holzheu" <holzheu(a)linux.vnet.ibm.com> wrote:
On Wed, 2010-02-10 at 10:08 -0500, Dave Anderson wrote:
> ----- "Michael Holzheu" <holzheu(a)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.
What kind of errors?
The attached patch would solve the problem (and eliminate most of the
probably redundant s390(x)_has_cpu() function.
I don't see what's being solved by the patch (not the s390x_get_smp_cpus
parts) -- does the "old" s390x_has_cpu() fail?
Even though the task is offline, the runqueue will still show its percpu
swapper task as the current task on that cpu.
Dave
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