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Re: [PATCH 4/4] rseq registration tests (v2)


On 4/4/19 4:04 PM, Mathieu Desnoyers wrote:
----- On Mar 27, 2019, at 5:21 PM, Carlos O'Donell codonell@redhat.com wrote:


+
+static int
+do_rseq_threads_test (int nr_threads)
+{
+  pthread_t th[nr_threads];
+  int i;
+  int result = 0;
+  pthread_attr_t at;
+
+  if (pthread_attr_init (&at) != 0)
+    {
+      FAIL_EXIT1 ("attr_init failed");
+    }
+
+  if (pthread_attr_setstacksize (&at, 1 * 1024 * 1024) != 0)

Why set it that small? Why set it at all?

I copied that boilerplate code from pre-existing glibc nptl tests,
in a true "monkey see, monkey do" fashion.

If we look at e.g. nptl/tst-fork1.c, those stack size limitations
were introduced by this commit:

commit 4d1a02efc1117763c67fe012642381e3106500cf
Author: Ulrich Drepper <drepper@redhat.com>
Date:   Sun Mar 7 10:40:53 2004 +0000

     Update.

     2004-03-07  Ulrich Drepper  <drepper@redhat.com>
* tst-once4.c: Remove unnecessary macro definition. * tst-mutex7.c (do_test): Limit thread stack size.
             * tst-once2.c (do_test): Likewise.
             * tst-tls3.c (do_test): Likewise.
             * tst-tls1.c (do_test): Likewise.
             * tst-signal3.c (do_test): Likewise.
             * tst-kill6.c (do_test): Likewise.
             * tst-key4.c (do_test): Likewise.
             * tst-join4.c (do_test): Likewise.
             * tst-fork1.c (do_test): Likewise.
             * tst-context1.c (do_test): Likewise.
             * tst-cond2.c (do_test): Likewise.
             * tst-cond10.c (do_test): Likewise.
             * tst-clock2.c (do_test): Likewise.
             * tst-cancel10.c (do_test): Likewise.
             * tst-basic2.c (do_test): Likewise.
             * tst-barrier4.c (do_test): Likewise.

And that commit does not state _why_ the thread stack size needs to
be limited. Nor on which architectures it matters, and what are the
parameters (e.g. number of threads) for which it matters.

In the case of rseq nptl tests, we spawn up to 50 threads in addition
to main.

Should we limit the thread stack size to 1MB ? That's indeed a good
question. Anyone would like to voice an opinion on the matter ?

Unless you have a strong reason for selecting 1MiB you should just
remove the boiler plate code and let the architecture default stack
sizes apply.

The above commit is a good example of a failure to provide a comment
that gives intent for the implementation and therefore you have no
idea why 1MiB was selected. Magic numbers should have comments, and
a patch like the one you reference would not be accepted today.

The only real worry we have with testing is thread reap rate which
seems to be slow in the kernel and sometimes we've seen the kernel
be unable to clone new threads because of this reason. Even then on
the worst architecture, hppa, I can create ~300 threads in a test
without any problems.

In summary:
- Remove the thread stack size setting.
- Optional: Send a distinct patch to cleanup the tests which have
  such boiler plate ;-)

--
Cheers,
Carlos.


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