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

[Alexandre Oliva <oliva@adacore.com>]

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
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