[patch v1] nptl: smarter not-parallel-ing

[Florian Weimer]

I did some benchmarks with this patch.

* Cold case.

Newly build source tree, “make subdirs=login check” (to populate the
testroot), followed by “make subdirs=nptl check”, timings for the
latter.  All tests were for x86-64, timings in seconds (wall clock).
Appropriate -j options supplied.

Before:

8-thread system:   399 399 397 400 398 400 400
256-thread system: 392 389 389 391 394 393 392

After:

8-thread system:   372 375 373 376 373 374 375
256-thread system: 357 357 356 361 358 358 357

* Warm case (incremental make).

Timings of “make subdirs=nptl check” after the first test suite run.
Appropriate -j options supplied.

After:

8-thread system:   1.541 1.014 1.025 1.016 1.030 1.029
256-thread system: 1.789 1.720 1.142 1.119 1.133 1.143

(I didn't bother to time the before case, considering these low
numbers.)

So it seems to me that performance-wise, this is a small overall win.

Thanks,
Florian