Re: git: 48bfd3597654 - main - Add nproc(1)

On 2/14/23, Jan Beich <jbeich@freebsd.org> wrote:
> Mateusz Guzik <mjguzik@gmail.com> writes:
>
>> On 2/14/23, Jan Beich <jbeich@freebsd.org> wrote:
>>
>>> Steffen Nurpmeso <steffen@sdaoden.eu> writes:
>>>
>>>>  |+     if (all_flag) {
>>>>  |+             cpus = sysconf(_SC_NPROCESSORS_ONLN);
>>>>
>>>> is not compatible -- note i like the FreeBSD way better, and maybe
>>>> someone should make a bug report even.
>>>>
>>>>   #?0|kent:$ getconf -a|grep NPROC
>>>>   _NPROCESSORS_CONF                  8
>>>>   _NPROCESSORS_ONLN                  4
>>>
>>> See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230330
>>>
>>> $ cpuset -l 2-5 -- getconf -a | fgrep _NPROC
>>> _NPROCESSORS_CONF: 8
>>> _NPROCESSORS_ONLN: 4
>>>
>>
>> As I mentioned previously in the thread, on linux these *don't* react
>> to cpusets (or tasksets in linux parlance):
>>
>> $ getconf -a | grep _NPROC
>> _NPROCESSORS_CONF                  4
>> _NPROCESSORS_ONLN                  4
>> $ taskset --cpu-list 0 getconf -a | grep _NPROC
>> _NPROCESSORS_CONF                  4
>> _NPROCESSORS_ONLN                  4
>>
>> consequently the patch proposed in the bz would introduce an
>> incompatibility with linux
>
> I've used kern.smp.cpus in old patches but found musl more practical.
> _SC_NPROCESSORS_ONLN is often used as a default number of jobs.
>

_SC_NPROCESSORS_ONLN is expected by real consumers to *not* be
affected by cpuset et al. See below for musl.

> glibc:
> - /sys/devices/system/cpu/online
> - /proc/stat

Neither of these are affected by taskset/cpuset.

> - sched_getaffinity

where is this used?

>
> musl:
> - sched_getaffinity
>

From the commit I don't think the choice was thought out whatsoever.
https://git.musl-libc.org/cgit/musl/commit/?id=9a7fac7934975338449b63eb4d0409b472251dfa

the code went from returning -1 to glibc-incompatible taskset-affected
count, and there are people complaining it breaks compat:

https://marc.info/?l=musl&w=2&r=1&s=_SC_NPROCESSORS_ONLN&q=b

> bionic:
> - /sys/devices/system/cpu/online
> - /proc/stat (Android < 7)
>

Neither of these are affected by taskset/cpuset.

All in all, the *current* state is broadly linux+glibc-compatible. The
one incompatibility concerns disabled HT, which on linux still reports
those cpu threads, while they are completely absent from the count on
freebsd. This is not taskset/cpuset-related.

-- 
Mateusz Guzik <mjguzik gmail.com>