Re: [musl] Re: sysconf(_SC_NPROCESSORS_CONF) broken.

[enh <enh@google.com>]

[-- Attachment #1: Type: text/plain, Size: 1727 bytes --]

On Tue, Jul 26, 2022 at 11:27 PM Rob Landley <rob@landley.net> wrote:

> On 7/27/22 01:28, Rob Landley wrote:
> > NPROCESSORS_CONF is supposed to show total processors, NPROCESSORS_ONLN
> shows
> > available processors using the tasket mask sched_getaffinity()).
> >
> > Musl is (uniquely) using the getaffinity() version for both. Neither
> glibc nor
> > bionic have that bug.
> >
> > Test: my laptop has 4 processors:
> >
> > $ taskset 7 nproc
> > 3
> > $ taskset 7 nproc --all
> > 4
> >
> > With musl, both show 3.
>
> P.S. According to strace, devuan's nprocs --all is reading sysfs:
>
> openat(AT_FDCWD, "/sys/devices/system/cpu",
> O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
> fstat(3, {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> getdents64(3, /* 22 entries */, 32768)  = 656
> getdents64(3, /* 0 entries */, 32768)   = 0
>
> At a guess, counting the cpu[0-9]* entries? I looked at bionic's source
> and it
> had a comment that x86 can use /proc/cpuinfo but arm only shows "available"
> processors there, not total processors...
>

yeah, bionic uses /sys/devices/system/cpu and
/sys/devices/system/cpu/online and we haven't had to touch that code since
we switched to doing it that way.

note that my claim in that comment about x86 may not be strictly correct.
i'm not sure i'd ever seen an x86 cpu powering down cores at that time, and
i'm not sure i have since either, but i assume that -- if such a thing is
possible -- they'd look the same as arm? i'd guess the comment should
actually say something like "on systems where cores are hotplugged", and
that this probably can happen on intel too (i'm guessing their recent
big.LITTLE equivalent will have brought this kind of thing to their world
too).


> Rob
>

[-- Attachment #2: Type: text/html, Size: 2430 bytes --]