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

[Shailabh Nagar]

Andrew Morton wrote:

> On Fri, 23 Jun 2006 15:07:28 -0700
> Jay Lan <jlan@xxxxxxxxxxxx> 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


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