Re: [musl] [BUG] sysconf implementing _SC_NPROCESSORS_(CONF|ONLN) incorrectly

[Szabolcs Nagy <nsz@port70.net>]

* Norbert Lange <nolange79@gmail.com> [2020-04-15 11:57:16 +0200]:
> I can't comment on whether glibc should be emulated. The point I am trying
> to make is that it might be better to let the compilation fail by default,
> or not provide the function at all.
> 
> The implementation right now doesn't seem sufficient (to put it midly) and
> it prevents detection and automatic fallbacks. For example trace-cmd would
> do this, and would work nicely - but instead it will gets musls
> implementation that's defeated by setting an affinity mask.

the point is that the glibc implementation is not sufficient either.

you don't get what you think you get as a result so you better off
to just always do the fallback.

identifying musl via a macro would be extremely bad in this case
since we are discussing to change the implementation and the
macro would not reflect that so a wrong default would be baked
into the source (which shows why it is a good idea not to provide
such a macro at all: most developers dont understand how to use
such macros and by now there would be a lot of broken musl
workarounds that are not relevant to the latest musl version).


> 
> Florian Weimer <fw@deneb.enyo.de> schrieb am Mi., 15. Apr. 2020, 11:50:
> 
> > * Norbert Lange:
> >
> > > How should  one deal with this?
> > > I understand that the semantics are vague, but given that musl now
> > > implements this
> > > function, it will make detection and fallback hard (especially as musl
> > > doesn't wants to be identified by the likes of macros).
> > >
> > > As it is now, just using the affinity mask definitely cant be useful,
> > > an application wanting that behavior should be patched to
> > > use that function directly.
> > > If musl would not define the _SC_NPROCESSORS_* macros (but still keep
> > > the implementation),
> > > this could be used for compile-time detection atleast. Enabling the
> > > current implementation would be
> > > just a matter of explicitly defining those macros.
> >
> > _SC_NPROCESSORS_* as implemented in glibc is bad because those values
> > are not adjusted by cgroups, so it can grossly overestimate available
> > resources.
> >
> > The cgroups interfaces themselves are not stable and very complicated.
> > I don't think it's a good idea to target them, especially not from
> > code that is expected to be linked statically into applications.
> >
> > Given that, I'm not sure that glibc's way is a significant
> > improvement.  musl should perhaps be changed to cope more gracefully
> > with a sched_getaffinity failure, though (by not reporting a UP
> > environment by accident).
> >