Re: Stap scripts to track task cpu changes

[William Cohen <wcohen at redhat dot com>]

Ankita Garg wrote:
> Hello William,
> 
>> Subject: Re: Stap scripts to track task cpu changes
>> Date: Thursday 09 Jul 2009
>> From: William Cohen <wcohen@redhat.com>
>> To: Ankita Garg <ankita@in.ibm.com>, systemtap@sources.redhat.com, 
>> prerna@linux.vnet.ibm.com
>>
>> Ankita Garg wrote:
>>> Hello folks,
>>>
>>> Quite sometime back, I was faced with a situation where I needed to
>>> track instances when a particular task was being migrated away from a
>>> cpu. I used the following trivial script. The tid needs to be passed as
>>> parameter. Sharing it, hoping it might be useful for some folks.
>> I would like to add these example to the systemtap examples. I have looked
>> through them and made some tweaks. For both of them I formatted them not to 
>> use
>> tabs so they can used in documentation.
>>
> 
> Thanks for making the required formatting changes.
> 
>> For chng_cpu.stp use the appropriate probe point, scheduler.cpu_on.  Why the
>> check for "tid() != 0"?
>>
>> For migrate.stp The check "$1 != 0" seemed to be unneeded. Was there a reason
>> for using $p->pid and kernel_string($p->comm) rather tid() and execname()?  
>> The
>> test is using "tid() == $1"
>>
> 
> True, the checks for tid() !=0 seem unneccesary. In migrate.stp (ie in
> function __migrate_task), the task being migrated need not be the
> current task. Hence, using $p->pid and $p->comm, where $p is the task being
> migrated.
>  
>> -Will

Hi Ankita,

A bit more tweaking on the examples. For the chng_cpu2.stp I moved the
assignment to threads associative array inside the if statement to avoid
overflowing the array. For migrate3.stp I made it take the executable name, so
that it works in a manner similar to chng_cpu2.stp. Was there multiple instances
of the same execname running one the machine or would using the execname be
sufficent for migrate.stp?

-Will

/*    Filename: chng_cpu.stp
 *      Author: Ankita Garg <ankita@in.ibm.com>
 * Description: Captures information on the number of times an executable
 * switches cpu
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Copyright (C) IBM Corp. 2009.  All Rights Reserved.
 *
 */

global threads

probe scheduler.cpu_on
{
  if ((threads[tid()] != cpu() && (execname() == @1))) {
    printf("\nthread %d (%s) context switched on cpu%d state: %d\n",
           tid(), execname(), cpu(), task_state(task_current()));
    print_stack(backtrace());
    threads[tid()] = cpu();
  }
}

/*    Filename: migrate.stp
 *      Author: Ankita Garg <ankita@in.ibm.com>
 * Description: Captures information on the migration of a thread
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Copyright (C) IBM Corp. 2009.  All Rights Reserved.
 *
*/

probe kernel.function("__migrate_task")
{
  comm = kernel_string($p->comm);
  if (comm == @1) {
    printf ("thread %d (%s) is migrating from %d to %d \n",
            $p->pid, comm, $src_cpu, $dest_cpu);
  }
}