Date: Wed, 15 Apr 2020 12:04:43 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Cc: Florian Weimer <fw@...eb.enyo.de>, Rich Felker <dalias@...c.org>, Norbert Lange <nolange79@...il.com> Subject: Re: [BUG] sysconf implementing _SC_NPROCESSORS_(CONF|ONLN) incorrectly * Norbert Lange <nolange79@...il.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@...eb.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). > >
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