Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

[Shailabh Nagar <nagar@watson.ibm.com>]

Andrew Morton wrote:

>On Fri, 23 Jun 2006 15:07:28 -0700
>Jay Lan <jlan@engr.sgi.com> wrote:
>
>  
>
>>>>       %ovhd of tgid on over off
>>>>       (higher is worse)
>>>>
>>>>Exit     User     Sys     Elapsed
>>>>Rate     Time     Time    Time
>>>>
>>>>2283      25.76  649.41   -0.14
>>>>1193     -10.53   88.81   -0.12
>>>>963      -11.90    3.28   -0.10
>>>>806       -8.54   -0.84    0.16
>>>>694       -4.41    2.38    0.03
>>>>   
>>>>        
>>>>
>>>Oh wow.  Something's gone quadratic there.
>>> 
>>>      
>>>
>>It was due to a loop in fill_tgid() when per-TG stats
>>data are assembled for netlink:
>>        do {
>>                rc = delayacct_add_tsk(stats, tsk);
>>                if (rc)
>>                        break;
>>
>>        } while_each_thread(first, tsk);
>>
>>and it is executed inside a lock.
>>Fortunately single threaded appls do not hit this code.
>>    
>>
>
>Am I reading this right?  We do that loop when each thread within the
>thread group exits?
>
Yes.

> How come?
>  
>
To get the sum of all per-tid data for threads that are currently alive.
This is returned to userspace with each thread exit.

>Is there some better lock we can use in there?  It only has to be
>threadgroup-wide rather than kernel-wide.
>  
>
The lock we're holding is the tasklist_lock. To go through all the 
threads of a thread group
thats the only lock that can protect integrity of while_each_thread afaics.

--Shailabh