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

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Andrew Morton wrote:

On Fri, 23 Jun 2006 15:07:28 -0700
Jay Lan <[email protected]> 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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