[PATCH] libstdc++: testsuite: call sched_yield for nonpreemptive targets

[PATCH] libstdc++: testsuite: call sched_yield for nonpreemptive targets

[Alexandre Oliva]

As in the gcc testsuite, systems without preemptive multi-threading
require sched_yield calls to be placed at points in which a context
switch might be needed to enable the test to complete.

Regstrapped on x86_64-linux-gnu, also tested with a cross to
aarch64-rtems6.  Ok to install?


for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/this_thread/60421.cc (test02): Call
	sched_yield.
---
 .../testsuite/30_threads/this_thread/60421.cc      |    1 +
 1 file changed, 1 insertion(+)

diff --git a/libstdc++-v3/testsuite/30_threads/this_thread/60421.cc b/libstdc++-v3/testsuite/30_threads/this_thread/60421.cc
index ad6f9aeffcc80..12dbeba1cc492 100644
--- a/libstdc++-v3/testsuite/30_threads/this_thread/60421.cc
+++ b/libstdc++-v3/testsuite/30_threads/this_thread/60421.cc
@@ -59,6 +59,7 @@ test02()
   while (!sleeping)
   {
     // Wait for the thread to start sleeping.
+    sched_yield ();
   }
   while (sleeping)
   {

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>

Re: [PATCH] libstdc++: testsuite: call sched_yield for nonpreemptive targets

[Jonathan Wakely]

On Tue, 21 Jun 2022 at 06:54, Alexandre Oliva via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
>
> As in the gcc testsuite, systems without preemptive multi-threading
> require sched_yield calls to be placed at points in which a context
> switch might be needed to enable the test to complete.

I'll try to remember that, but will probably forget. Is this really
the only affected test?

>
> Regstrapped on x86_64-linux-gnu, also tested with a cross to
> aarch64-rtems6.  Ok to install?

OK, thanks.

Re: [PATCH] libstdc++: testsuite: call sched_yield for nonpreemptive targets

[Alexandre Oliva]

On Jun 21, 2022, Jonathan Wakely <jwakely@redhat.com> wrote:

> On Tue, 21 Jun 2022 at 06:54, Alexandre Oliva via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
>> 
>> 
>> As in the gcc testsuite, systems without preemptive multi-threading
>> require sched_yield calls to be placed at points in which a context
>> switch might be needed to enable the test to complete.

> I'll try to remember that, but will probably forget. Is this really
> the only affected test?

Yeah, the only one in libstdc++-v3/testsuite, as of gcc-11, which is
what I've focused on for this project; I haven't gone through all fails
on master to make sure they're unrelated.  (This is holding up some
filesystem-related patches, whose tests are failing for reasons not
present in gcc-11, and some of the forward-ports needed significant
rewriting.)

libstdc++ has plenty of threading tests, but they involve sleeping or
otherwise blocking, which offers a context switch opportunity even on
non-preemptive multithreading systems.  It's busy waits that require a
sched_yield.  The gcc testsuite has a handful of those; covered by a
separate patch also posted last night.

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>