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Date: Fri, 23 Jun 2006 16:47:43 -0700
From: Andrew Morton <akpm@osdl.org>
To: Jay Lan <jlan@engr.sgi.com>
Cc: nagar@watson.ibm.com, balbir@in.ibm.com, csturtiv@sgi.com,
       linux-kernel@vger.kernel.org
Subject: Re: [Patch][RFC]  Disabling per-tgid stats on task exit in
 taskstats
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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?  How come?

Is there some better lock we can use in there?  It only has to be
threadgroup-wide rather than kernel-wide.