[PATCH] manual: Document lack of conformance of sched_* functions [BZ #14829]

[Florian Weimer]

* Carlos O'Donell:

>> 2018-12-07  Florian Weimer  <fweimer@redhat.com>
>> 
>> 	[BZ #14829]
>> 	* manual/resource.texi (Basic Scheduling Functions): Add
>> 	portability note.  Change process to task throughout the section.
>> 	Remove incorrect comment about sched_yield as it affects
>> 	tasks/threads, not entire processes.
>
> I think we need a *big* comment in sysdeps/unix/sysv/linux/bits/posix_opt.h
> around the define of _POSIX_PRIORITY_SCHEDULING which says:
>
> /* On Linux we do not conform to the POSIX requirements for setting
>    _POSIX_PRIORITY_SCHEDULING, and it should be set to -1, but it has
>    been enabled for so long that we cannot risk setting it to -1 without
>    serious issues arising with existing applications, so we leave it enabled
>    even though on Linux the APIs all take thread IDs.  Please see bug 14829.  */
>
> What do you think?

I think that is a separate discussion.

We could stop defining _POSIX_PRIORITY_SCHEDULING to -1, but still
provide the corresponding definitions and declarations under
_GNU_SOURCE.  Whether that's feasible requires substantial additional
research.

I'm fine with adding a comment to
sysdeps/unix/sysv/linux/bits/posix_opt.h along the lines you suggested,
maybe:

/* Priority scheduling is not supported with the correct semantics, but
   GNU/Linux applications expect that the corresponding interfaces are
   available, even though the semantics do not meet the POSIX
   requirements.  */
#define	_POSIX_PRIORITY_SCHEDULING	200809L

Or we could use the text you proposed.

>> diff --git a/manual/resource.texi b/manual/resource.texi
>> index 8bc2a803d4..f02192475a 100644
>> --- a/manual/resource.texi
>> +++ b/manual/resource.texi
>> @@ -750,6 +750,14 @@ policy, if anything, only fine tunes the effect of that priority.
>>  
>>  The symbols in this section are declared by including file @file{sched.h}.
>>  
>> +@strong{Portability Note:} In POSIX, the @code{pid_t} arguments of the
>> +functions below refer to process IDs.  On Linux, they are actually
>> +thread IDs, and control how specific threads are scheduled with
>> +regards to the entire system.  The resulting behavior does not conform
>> +to POSIX.  This is why the following description refers to tasks and
>> +tasks IDs, and not processes and process IDs.
>> +@c https://sourceware.org/bugzilla/show_bug.cgi?id=14829
>
> OK.
>
> Should we also mention that PTHREAD_SCOPE_PROCESS is entirely unsupported by
> glibc on Linux?

Wouldn't that be something for the documentation of
pthread_attr_setscope, which does not exist yet?

Thanks,
Florian