OK. We have measured the nptl perf test for init namespace.
Summary - flat model is very light, Suka's patches break the 
kernel performance event when CONFIG_PID_NS is off.

                 |    perf, s   | perf loss  |
-----------------+--------------+------------+
2.6.22-rc4-mm2   | 13.79 ± 0.13 |    ---     |
                 |              |            |
suka  + PID_NS=n | 14.07 ± 0.13 |    2.0%    |
suka  + PID_NS=y | 14.06 ± 0.08 |    2.0%    |
                 |              |            |
pavel + PID_NS=n | 13.80 ± 0.07 |    0.0%    |
pavel + FLAT     | 13.80 ± 0.07 |    0.0%    |
pavel + MULTI    | 14.28 ± 0.08 |    3.5%    |
-----------------+--------------+------------+

I do believe that Suka's hierarchical model is better than mine,
but my point is: let's support the flat model as well.

Thanks,
Pavel.
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